configure: allow building with python3

Pretty much all of the scripts are python2+3 compatible.
Check and allow using python3, while adjusting the PYTHON2 refs.

Note:
 - python3.4 is used as it's the earliest supported version
 - python3 chosen prior to python2

Signed-off-by: Emil Velikov <emil.velikov@collabora.com>
Acked-by: Eric Engestrom <eric.engestrom@intel.com>

src/mapi/Makefile.am

@@ -59,7 +59,7 @@ AM_CPPFLAGS =							\
 include Makefile.sources
 
 MKDIR_GEN = $(AM_V_at)$(MKDIR_P) $(@D)
-PYTHON_GEN = $(AM_V_GEN)$(PYTHON2) $(PYTHON_FLAGS)
+PYTHON_GEN = $(AM_V_GEN)$(PYTHON) $(PYTHON_FLAGS)
 
 glapi_gen_mapi_deps := \
 	mapi_abi.py \

src/mapi/glapi/gen/Makefile.am

@@ -233,7 +233,7 @@ COMMON = $(API_XML) \
 
 COMMON_GLX = $(COMMON) glX_API.xml glX_XML.py glX_proto_common.py
 
-PYTHON_GEN = $(AM_V_GEN)$(PYTHON2) $(PYTHON_FLAGS)
+PYTHON_GEN = $(AM_V_GEN)$(PYTHON) $(PYTHON_FLAGS)
 
 ######################################################################