Our configure test checks the presence of a Python2, but then uses
"python" as interpreter, which is a problem nowadays as the default
python is set to be Python 3 on some distributions (and this will be
more and more the case). So GIMP may end up trying to run our plug-ins
through Python 3 (which would fail) even if Python 2 is present.
Now AM_PATH_PYTHON2() m4 macro will set $PYTHON to a more accurate
Python version as priority.
Similarly let's use "python2" in the binfmt string for extension search.
(cherry picked from commit 03ea9cac54)
42b13216fc