on OSX, cmake choose libxml2 from Gstreamer package when it is present rather than the one installed with brew
then it fails to build because of missing header

I fixed this issue by manually setting Cmake variable :

LIBXML_INCLUDE_DIR=/usr/local/opt/libxml2/include/libxml2
LIBXML_LIBRARIES=/usr/local/opt/libxml2/lib/libxml2.dylib
LIBXML_XMLLINT_EXECUTABLE=/usr/local/opt/libxml2/bin/xmllint

Obviously the paths could be different depending on where you install it.

Also, I'm not sure this bug should be fixed, if so mark it as won't fix and leave it open for the record.