libxml2: disable python usage on macos

Matches the configuration on every other platform.

Avoids libxml2/autotools from using the python framework within XCode itself which now has a helpful error.

libtool: link: clang -Wl,-undefined -Wl,dynamic_lookup -o .libs/libxml2mod.so -bundle  .libs/libxml.o .libs/libxml2-py.o .libs/types.o   ../.libs/libxml2.dylib -L/Library/Frameworks/GStreamer.framework/Versions/1.0/lib -lpthread /Library/Frameworks/GStreamer.framework/Versions/1.0/lib/libz.dylib -liconv -lm -L/System/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/config -lpython2.7 -ldl -framework CoreFoundation  -arch x86_64 -m64 -mmacosx-version-min=10.11 -isysroot /Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX12.0.sdk -g -O2 -Wl,-headerpad_max_install_names -arch x86_64 -m64 -Wl,-arch -Wl,x86_64 -mmacosx-version-min=10.11 -isysroot /Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX12.0.sdk   -framework CoreFoundation
ld: cannot link directly with dylib/framework, your binary is not an allowed client of /Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX12.0.sdk/System/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/config/libpython2.7.tbd for architecture x86_64