Rewrite automake rules for internationalisation files.

I got fed up with battling against the Makefile which
gettext provides.  So I rolled my own instead.  This change provides
the following advantages over previous situation:

* It doesn't use recursive calls to $(MAKE).

* Merged and unmerged po files are properly seperated into
  the buildir and sourcedir respectively.  Consequently,
  po files get updated only when we really want them to be,
  and not (potentially) on every commit.

* The .pot file (and consequently the .po and .gmo files) now
  properly depend upon all the distributed source files.  There
  is no need to run "make -f Smake" to make it aware of new files.

* There's no more need of "timestamp" files.

* Potentially it's easier to add per-source-file options
  when generating the .pot file. (and thus avoid warnings about
  unrecognised extentions.