I verified that the include files are in /usr/local/opt/libxml2/include/libxml2/libxml.
I tried adding "-I/usr/local/opt/libxml2/include" and "-I/usr/local/opt/libxml2/include/libxml2" to CFLAGS and to CPPFLAGS, neither of which helped.
I verified that the include files are in /usr/local/ opt/libxml2/ include/ libxml2/ libxml.
I tried adding "-I/usr/ local/opt/ libxml2/ include" and "-I/usr/ local/opt/ libxml2/ include/ libxml2" to CFLAGS and to CPPFLAGS, neither of which helped.