src translations follow-up: Add a utility for generating a po.h header

[MichaelChirico]

For packages that are not equipped yet at all for translation, they'll need to define the translation macro/includes to get translation-ready.

It should be easy to generate a simple po.h file, but the devil's in the details:

1. Detecting that po.h is needed in the first place (should be relatively simple)
2. Deciding where exactly to insert #include "po.h"

[MichaelChirico]

I'm wavering on whether this is needed for initial release. Currently coming down on "no".