lxml does not build with Xcode 9

[Barry Warsaw]

lxml does not build from tarball with Xcode 9.0.1, macOS 10.12.6:

$ python3 -mvenv /tmp/lxml
$ source /tmp/lxml/bin/activate
$ pip install lxml --no-binary :all:
...
    clang -bundle -undefined dynamic_lookup build/temp.macosx-10.12-x86_64-3.6/src/lxml/etree.o -L/Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX10.13.sdk/usr/lib -L/usr/local/lib -L/usr/local/opt/openssl/lib -L/usr/local/opt/sqlite/lib -lxslt -lexslt -lxml2 -lz -lm -o build/lib.macosx-10.12-x86_64-3.6/lxml/etree.cpython-36m-darwin.so
    ld: file not found: /usr/lib/system/libsystem_darwin.dylib for architecture x86_64
    clang: error: linker command failed with exit code 1 (use -v to see invocation)
    error: command 'clang' failed with exit status 1

While it's very possible that Xcode 9 has a bug, since libsystem_darwin.dylib does not exist, Apple has yet to acknowledge or fix it. There's additional discussion here:

https://forums.developer.apple.com/thread/87829

which provides for a workaround that lxml itself could implement.

[scoder (scoder)]

Mac build support is usually done by mac users, but I take pull requests.

[Saulius Zemaitaitis (e-saulius)]

Minimal pull request to make builds work on macosx 10.12 with XCode 9: https://github.com/lxml/lxml/pull/258