From [http://tagoh.bitbucket.org/liblangtag/]. An interface for BCP47 language tags.

Note that (as far a I recall) liblangtag is the only "bundled"
(3rd-party) library we have that we configure and build also for MSVC
using an autotools+libtool based mechanism. That is rather fragile and
works maybe mostly by accident. So be careful not to break it.
