configure.ac: search for and set PYTHON3

src/intel/genxml/gen_pack_header.py requires python3.

v2: check for python3.5 as well

Signed-off-by: Jonathan Gray <jsg@jsg.id.au>
Reviewed-by: Emil Velikov <emil.velikov@collabora.com>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
1 file changed