Revert "configure: allow building with python3"

This reverts commit ae7898dfdb.

Turns out the python scripts are _not_ fully python 3 compatible.
As Ilia reported using get_xmlpool.py with LANG=C produces some weird
output - see the link for details.

Even though the issue was spotted with the autoconf build, it exposes a
genuine problem with the script (and lack of lang handling of the meson
build.)

https://lists.freedesktop.org/archives/mesa-dev/2018-August/203508.html

m4/ax_check_python_mako_module.m4

@@ -52,7 +52,7 @@ else:
     ver_req = distutils.version.LooseVersion('$1')
     ver_act = distutils.version.LooseVersion(mako.__version__)
     sys.exit(int(ver_req > ver_act))
-    " | $PYTHON -
+    " | $PYTHON2 -
 
     if test $? -ne 0 ; then
        AC_MSG_RESULT(no)