m4macros, plug-ins: run more accurately the "python2" interpreter.
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.
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