glib configure script uses by default python2.7 to substitute in scripts,
but it also supports --with-python argument. Since the scripts support
python2 or python3, we can use the full path of the python used by
buildout. This will sometimes create shebang lines like:

    #!/usr/bin/env /path/to/slapos/python3/bin/python

but this also work fine.

This replaces the following commits:
 - 7581b668 (component/glib: remove bad file bin/gtester-report after compilation, 2020-08-07)
 - 9c47f828 (componnent/gtk-2: Add python2.7 as the dependency of pango, 2022-08-18)