Move po gen to cmake

This takes a stab at fixing #902 by moving the language-building bits into CMake.

I'm by no means a gettext/.po guru, so I'd appreciate any feedback anyone has to offer--especially from @camponez, @bfredl, and @fwalch who have updated translations recently.  Make sure you save your work if you're in the middle of editing though!  The new update mechanism does shuffle the contents around the first time.

Also, I'd like feedback on the names of some of the targets.  For instance, `update-pt_BR` will update the pt_BR .po file.  I was thinking it should probably be namespaced a little more, like `update-po-pt_BR`.

Also, I don't have a `check-po` target that checks all translations.  I avoided it for now because there are some issues present, so the build would just end up failing on the first issue it encountered.  I also didn't try making a report either since it wasn't originally available in the Makefile and I was avoiding trying to do too much at once.