As build-requirement, we either require
  - python2 with python-gobject-base
  - python3 with python3-gobject-base
Previously, we would require that a plain `python` gives the desired
interpreter version.

If somebody's "/usr/bin/env python" however points to a different
python version, there was no easy way to change it -- aside
resetting the $PATH variable to some desired "python" binary.

Now, you can specify it during configure:

  ./configure PYTHON=python3 ...

This especially matters, if you only have python3-gobject-base
installed, you /usr/bin/python is a symlink to python2.

https://bugzilla.gnome.org/show_bug.cgi?id=775768