1/ First I realize we were still using AM_PATH_PYTHON() and not
   AM_PATH_PYTHON3().
2/ We don't need to check anymore for pygtk, pycairo, nor do we need
   Python headers. Everything is now taken care of by GObject
   Introspection and we have nothing else to build GIMP-side.
3/ Change --enable-python into --with-python. We don't actually build
   anything now. There is no Python support to enable or disable
   anymore. We simply install Python plug-ins or not. The 3 values are
   "yes" (default, configure breaks if a Python 3 interpreter is not
   found), "no" (Python plug-ins are not installed, not recommended) and
   "force" (install Python plug-ins anyway even if an interpreter is not
   found; this can be useful especially for cross-compilation since
   Python is not useful at build-time anymore).

   Note that --with-python=force, if an interpreter is not found, the
   file `pygimp.interp` won't be installed. This is not a problem at all
   if the interpreter can be found at runtime the standard way.
   Otherwise packagers should add themselves a pygimp.interp file with
   the known interpreter path.