Figure out the internal ./intl

Right now, gettext is configured to use an internal `intl`, which is all nice and dandy, if you get a tarball that actually contains the library.
Otherwise, you may need to run `gettextize --intl` yourself, which, to add insult to injury, is actually  deprecated. `autoreconf` will just invoke `autopoint`, which does not necessarily add `./intl`.
This makes it unnecessarily hard for new folk to modify and/or contribute.

Possible solutions:
- Just commit `./intl`
- Remove support for internal gettext by using `AM_GNU_GETTEXT([external])`  and remove remaining `./intl`stuff from configure.ac and Makefile.am.
- Leave as is, but document `gettextize --intl`.