https://bugs.launchpad.net/lxml/+bug/1614603 fixed an issue on Linux, but broke OS X.
`pip install lxml==3.6.3` fails with this error:
##
Building wheels for collected packages: lxml
Running setup.py bdist_wheel for lxml
Complete output from command /Users/danchenshao/Uber/.pyenvs/foober/bin/python2.7 -c "import setuptools;__file__='/private/var/folders/6z/q0_8zdvj4kj9tr1l31q1mx580000gn/T/pip-build-FmzRyj/lxml/setup.py';exec(compile(open(__file__).read().replace('\r\n', '\n'), __file__, 'exec'))" bdist_wheel -d /var/folders/6z/q0_8zdvj4kj9tr1l31q1mx580000gn/T/tmpAbN1KIpip-wheel-:
Building lxml version 3.6.3.
Building without Cython.
Using build configuration of libxslt 1.1.28
running bdist_wheel
running build
running build_py
creating build
creating build/lib.macosx-10.10-x86_64-2.7
creating build/lib.macosx-10.10-x86_64-2.7/lxml
copying src/lxml/__init__.py -> build/lib.macosx-10.10-x86_64-2.7/lxml
copying src/lxml/_elementpath.py -> build/lib.macosx-10.10-x86_64-2.7/lxml
copying src/lxml/builder.py -> build/lib.macosx-10.10-x86_64-2.7/lxml
copying src/lxml/cssselect.py -> build/lib.macosx-10.10-x86_64-2.7/lxml
copying src/lxml/doctestcompare.py -> build/lib.macosx-10.10-x86_64-2.7/lxml
copying src/lxml/ElementInclude.py -> build/lib.macosx-10.10-x86_64-2.7/lxml
copying src/lxml/pyclasslookup.py -> build/lib.macosx-10.10-x86_64-2.7/lxml
copying src/lxml/sax.py -> build/lib.macosx-10.10-x86_64-2.7/lxml
copying src/lxml/usedoctest.py -> build/lib.macosx-10.10-x86_64-2.7/lxml
creating build/lib.macosx-10.10-x86_64-2.7/lxml/includes
copying src/lxml/includes/__init__.py -> build/lib.macosx-10.10-x86_64-2.7/lxml/includes
creating build/lib.macosx-10.10-x86_64-2.7/lxml/html
copying src/lxml/html/__init__.py -> build/lib.macosx-10.10-x86_64-2.7/lxml/html
copying src/lxml/html/_diffcommand.py -> build/lib.macosx-10.10-x86_64-2.7/lxml/html
copying src/lxml/html/_html5builder.py -> build/lib.macosx-10.10-x86_64-2.7/lxml/html
copying src/lxml/html/_setmixin.py -> build/lib.macosx-10.10-x86_64-2.7/lxml/html
copying src/lxml/html/builder.py -> build/lib.macosx-10.10-x86_64-2.7/lxml/html
copying src/lxml/html/clean.py -> build/lib.macosx-10.10-x86_64-2.7/lxml/html
copying src/lxml/html/defs.py -> build/lib.macosx-10.10-x86_64-2.7/lxml/html
copying src/lxml/html/diff.py -> build/lib.macosx-10.10-x86_64-2.7/lxml/html
copying src/lxml/html/ElementSoup.py -> build/lib.macosx-10.10-x86_64-2.7/lxml/html
copying src/lxml/html/formfill.py -> build/lib.macosx-10.10-x86_64-2.7/lxml/html
copying src/lxml/html/html5parser.py -> build/lib.macosx-10.10-x86_64-2.7/lxml/html
copying src/lxml/html/soupparser.py -> build/lib.macosx-10.10-x86_64-2.7/lxml/html
copying src/lxml/html/usedoctest.py -> build/lib.macosx-10.10-x86_64-2.7/lxml/html
creating build/lib.macosx-10.10-x86_64-2.7/lxml/isoschematron
copying src/lxml/isoschematron/__init__.py -> build/lib.macosx-10.10-x86_64-2.7/lxml/isoschematron
copying src/lxml/lxml.etree.h -> build/lib.macosx-10.10-x86_64-2.7/lxml
copying src/lxml/lxml.etree_api.h -> build/lib.macosx-10.10-x86_64-2.7/lxml
copying src/lxml/includes/c14n.pxd -> build/lib.macosx-10.10-x86_64-2.7/lxml/includes
copying src/lxml/includes/config.pxd -> build/lib.macosx-10.10-x86_64-2.7/lxml/includes
copying src/lxml/includes/dtdvalid.pxd -> build/lib.macosx-10.10-x86_64-2.7/lxml/includes
copying src/lxml/includes/etreepublic.pxd -> build/lib.macosx-10.10-x86_64-2.7/lxml/includes
copying src/lxml/includes/htmlparser.pxd -> build/lib.macosx-10.10-x86_64-2.7/lxml/includes
copying src/lxml/includes/relaxng.pxd -> build/lib.macosx-10.10-x86_64-2.7/lxml/includes
copying src/lxml/includes/schematron.pxd -> build/lib.macosx-10.10-x86_64-2.7/lxml/includes
copying src/lxml/includes/tree.pxd -> build/lib.macosx-10.10-x86_64-2.7/lxml/includes
copying src/lxml/includes/uri.pxd -> build/lib.macosx-10.10-x86_64-2.7/lxml/includes
copying src/lxml/includes/xinclude.pxd -> build/lib.macosx-10.10-x86_64-2.7/lxml/includes
copying src/lxml/includes/xmlerror.pxd -> build/lib.macosx-10.10-x86_64-2.7/lxml/includes
copying src/lxml/includes/xmlparser.pxd -> build/lib.macosx-10.10-x86_64-2.7/lxml/includes
copying src/lxml/includes/xmlschema.pxd -> build/lib.macosx-10.10-x86_64-2.7/lxml/includes
copying src/lxml/includes/xpath.pxd -> build/lib.macosx-10.10-x86_64-2.7/lxml/includes
copying src/lxml/includes/xslt.pxd -> build/lib.macosx-10.10-x86_64-2.7/lxml/includes
copying src/lxml/includes/etree_defs.h -> build/lib.macosx-10.10-x86_64-2.7/lxml/includes
copying src/lxml/includes/lxml-version.h -> build/lib.macosx-10.10-x86_64-2.7/lxml/includes
creating build/lib.macosx-10.10-x86_64-2.7/lxml/isoschematron/resources
creating build/lib.macosx-10.10-x86_64-2.7/lxml/isoschematron/resources/rng
copying src/lxml/isoschematron/resources/rng/iso-schematron.rng -> build/lib.macosx-10.10-x86_64-2.7/lxml/isoschematron/resources/rng
creating build/lib.macosx-10.10-x86_64-2.7/lxml/isoschematron/resources/xsl
copying src/lxml/isoschematron/resources/xsl/RNG2Schtrn.xsl -> build/lib.macosx-10.10-x86_64-2.7/lxml/isoschematron/resources/xsl
copying src/lxml/isoschematron/resources/xsl/XSD2Schtrn.xsl -> build/lib.macosx-10.10-x86_64-2.7/lxml/isoschematron/resources/xsl
creating build/lib.macosx-10.10-x86_64-2.7/lxml/isoschematron/resources/xsl/iso-schematron-xslt1
copying src/lxml/isoschematron/resources/xsl/iso-schematron-xslt1/iso_abstract_expand.xsl -> build/lib.macosx-10.10-x86_64-2.7/lxml/isoschematron/resources/xsl/iso-schematron-xslt1
copying src/lxml/isoschematron/resources/xsl/iso-schematron-xslt1/iso_dsdl_include.xsl -> build/lib.macosx-10.10-x86_64-2.7/lxml/isoschematron/resources/xsl/iso-schematron-xslt1
copying src/lxml/isoschematron/resources/xsl/iso-schematron-xslt1/iso_schematron_message.xsl -> build/lib.macosx-10.10-x86_64-2.7/lxml/isoschematron/resources/xsl/iso-schematron-xslt1
copying src/lxml/isoschematron/resources/xsl/iso-schematron-xslt1/iso_schematron_skeleton_for_xslt1.xsl -> build/lib.macosx-10.10-x86_64-2.7/lxml/isoschematron/resources/xsl/iso-schematron-xslt1
copying src/lxml/isoschematron/resources/xsl/iso-schematron-xslt1/iso_svrl_for_xslt1.xsl -> build/lib.macosx-10.10-x86_64-2.7/lxml/isoschematron/resources/xsl/iso-schematron-xslt1
copying src/lxml/isoschematron/resources/xsl/iso-schematron-xslt1/readme.txt -> build/lib.macosx-10.10-x86_64-2.7/lxml/isoschematron/resources/xsl/iso-schematron-xslt1
running build_ext
building 'lxml.etree' extension
creating build/temp.macosx-10.10-x86_64-2.7
creating build/temp.macosx-10.10-x86_64-2.7/src
creating build/temp.macosx-10.10-x86_64-2.7/src/lxml
clang -fno-strict-aliasing -fno-common -dynamic -g -O2 -DNDEBUG -g -fwrapv -O3 -Wall -Wstrict-prototypes -I/usr/include/libxml2 -Isrc/lxml/includes -I/usr/local/Cellar/python/2.7.11/Frameworks/Python.framework/Versions/2.7/include/python2.7 -c src/lxml/lxml.etree.c -o build/temp.macosx-10.10-x86_64-2.7/src/lxml/lxml.etree.o -w -flat_namespace
clang -bundle -undefined dynamic_lookup build/temp.macosx-10.10-x86_64-2.7/src/lxml/lxml.etree.o -lxslt -lexslt -lrt -lxml2 -lz -lm -o build/lib.macosx-10.10-x86_64-2.7/lxml/etree.so
ld: library not found for -lrt
clang: error: linker command failed with exit code 1 (use -v to see invocation)
error: command 'clang' failed with exit status 1
----------------------------------------
Failed building wheel for lxml
Failed to build lxml
Installing collected packages: lxml
Found existing installation: lxml 3.6.1
Uninstalling lxml-3.6.1:
Successfully uninstalled lxml-3.6.1
Running setup.py install for lxml
Complete output from command /Users/danchenshao/Uber/.pyenvs/foober/bin/python2.7 -c "import setuptools, tokenize;__file__='/private/var/folders/6z/q0_8zdvj4kj9tr1l31q1mx580000gn/T/pip-build-FmzRyj/lxml/setup.py';exec(compile(getattr(tokenize, 'open', open)(__file__).read().replace('\r\n', '\n'), __file__, 'exec'))" install --record /var/folders/6z/q0_8zdvj4kj9tr1l31q1mx580000gn/T/pip-MneQsb-record/install-record.txt --single-version-externally-managed --compile --install-headers /Users/danchenshao/Uber/.pyenvs/foober/bin/../include/site/python2.7/lxml:
Building lxml version 3.6.3.
Building without Cython.
Using build configuration of libxslt 1.1.28
running install
running build
running build_py
copying src/lxml/includes/lxml-version.h -> build/lib.macosx-10.10-x86_64-2.7/lxml/includes
running build_ext
building 'lxml.etree' extension
clang -fno-strict-aliasing -fno-common -dynamic -g -O2 -DNDEBUG -g -fwrapv -O3 -Wall -Wstrict-prototypes -I/usr/include/libxml2 -Isrc/lxml/includes -I/usr/local/Cellar/python/2.7.11/Frameworks/Python.framework/Versions/2.7/include/python2.7 -c src/lxml/lxml.etree.c -o build/temp.macosx-10.10-x86_64-2.7/src/lxml/lxml.etree.o -w -flat_namespace
clang -bundle -undefined dynamic_lookup build/temp.macosx-10.10-x86_64-2.7/src/lxml/lxml.etree.o -lxslt -lexslt -lrt -lxml2 -lz -lm -o build/lib.macosx-10.10-x86_64-2.7/lxml/etree.so
ld: library not found for -lrt
clang: error: linker command failed with exit code 1 (use -v to see invocation)
error: command 'clang' failed with exit status 1
----------------------------------------
Rolling back uninstall of lxml
Command "/Users/danchenshao/Uber/.pyenvs/foober/bin/python2.7 -c "import setuptools, tokenize;__file__='/private/var/folders/6z/q0_8zdvj4kj9tr1l31q1mx580000gn/T/pip-build-FmzRyj/lxml/setup.py';exec(compile(getattr(tokenize, 'open', open)(__file__).read().replace('\r\n', '\n'), __file__, 'exec'))" install --record /var/folders/6z/q0_8zdvj4kj9tr1l31q1mx580000gn/T/pip-MneQsb-record/install-record.txt --single-version-externally-managed --compile --install-headers /Users/danchenshao/Uber/.pyenvs/foober/bin/../include/site/python2.7/lxml" failed with error code 1 in /private/var/folders/6z/q0_8zdvj4kj9tr1l31q1mx580000gn/T/pip-build-FmzRyj/lxml
##
From quick googling, it looks like OSX doens't need the "-lrt" flag at all. I found several PRs on other projects that simply removed that flag when compiling on OSX and that fixed it for them
