Since nobody can agree on adding `printf_l` to the standard, we have to
do this using OS-dependent mechanisms. On glibc, the best hack-around is
to use `uselocale` to set the thread-specific locale. This requires some
roundabout locale initialization, but works.

For other platforms (e.g. BSD and Win32), there are saner methods of
accomplishing this, which is why it's inside osdep/.