build: Fix build on systems where /usr/bin/python isn't python 2.

configure.ac sets up a PYTHON2 variable, which is what we want
AX_PYTHON_MODULE to use (since we only use Python 2 for now).

NOTE: This is a candidate for the 9.0 branch.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=31598
Reviewed-by: Matt Turner <mattst88@gmail.com>
Reviewed-by: Andreas Boll <andreas.boll.dev@gmail.com>

m4/ax_python_module.m4

@@ -25,13 +25,13 @@
 
 AU_ALIAS([AC_PYTHON_MODULE], [AX_PYTHON_MODULE])
 AC_DEFUN([AX_PYTHON_MODULE],[
-    if test -z $PYTHON;
+    if test -z $PYTHON2;
     then
-        PYTHON="python"
+        PYTHON2="python"
     fi
-    PYTHON_NAME=`basename $PYTHON`
+    PYTHON_NAME=`basename $PYTHON2`
     AC_MSG_CHECKING($PYTHON_NAME module: $1)
-	$PYTHON -c "import $1" 2>/dev/null
+	$PYTHON2 -c "import $1" 2>/dev/null
 	if test $? -eq 0;
 	then
 		AC_MSG_RESULT(yes)