Unfortunately the dist target requires pygtk.
Note that even adding --disable-python to DISTCHECK_CONFIGURE_FLAGS
doesn't help as pygtk-codegen-2.0 is needed to generate various .defs
file added to the DIST.
Yet pygtk is absent from Debian testing nowadays because it is too old
and unmaintained. So let's build pygtk ourselves.

Among the few tricks to make pygtk build fine:
- force Python2 by setting PYTHON env variable (detect Python 3
  otherwise).
- patch to allow building against latest Pango.