build: allow to override path to python without failing version checks

https://www.gnu.org/software/automake/manual/html_node/Python.html

If we configure with: ./configure PYTHON=/usr/bin/python2
the current call to AM_PATH_PYTHON will fail to detect version >= 3.3

As explained in the AM_PATH_PYTHON documentation:

"If the PYTHON variable is set when AM_PATH_PYTHON is called,
then that will be the only Python interpreter that is tried."

and AM_PATH_PYTHON will fail with error, without moving to
action-if-not-found, because it is found, but in the incorrect version.

Signed-off-by: Fabio M. Di Nitto <fdinitto@redhat.com>
Reviewed-by: Jan Pokorný <jpokorny@redhat.com>