po/Makefile.in.in: intltool compatibility fixes

Our infrastructure for updating the glib20.pot file was incompatible
with intltool in a couple of ways:

 - we didn't set the Report-Msgid-Bugs-To to

 - we used paths relative to the $(top_srcdir), rather than po/

This resulted in spamming of pointless changes to the .po files whenever
'make update-po' was run (since all of the translators are using
intltool-based tools).