libxml2: add support for building host library with python support

Patch based on the initial work of Will Wagner (Thanks Will).
libxml2 host library with python support is required to build mesa3d (7.10.1)

Signed-off-by: Vellemans Noel <noel.vellemans@visionbms.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>