Fix build on systems where "python" is python 3.

First, it changes autoconf to use a "python2" binary when available,
rather than plain "python" (which is ambiguous).  Secondly, it changes
the Makefiles to use $(PYTHON) $(PYTHON_FLAGS) rather than calling
python directly.

Signed-off-by: Xavier Chantry <chantry.xavier@gmail.com>
Signed-off-by: Matthew William Cox <matt@mattcox.ca>
Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
diff --git a/configure.ac b/configure.ac
index 364ee03..d6f15b5 100644
--- a/configure.ac
+++ b/configure.ac
@@ -30,6 +30,7 @@
 AC_PROG_CC
 AC_PROG_CXX
 AC_CHECK_PROGS([MAKE], [gmake make])
+AC_CHECK_PROGS([PYTHON2], [python2 python])
 AC_PATH_PROG([MKDEP], [makedepend])
 AC_PATH_PROG([SED], [sed])
 
@@ -1691,6 +1692,8 @@
 echo "        CFLAGS:          $cflags"
 echo "        CXXFLAGS:        $cxxflags"
 echo "        Macros:          $defines"
+echo ""
+echo "        PYTHON2:         $PYTHON2"
 
 echo ""
 echo "        Run '${MAKE-make}' to build Mesa"