bstr.c doesn't really deserve its own directory, and compat had just
a few files, most of which may as well be in osdep. There isn't really
any justification for these extra directories, so get rid of them.
The compat/libav.h was empty - just delete it. We changed our approach
to API compatibility, and will likely not need it anymore.
Before this commit, SRT demuxing and display actually happened to work
on Libav. But it was using the libavcodec srt converter (which is
essentially unmaintained in Libav), and timing postprocessing didn't
work. For some background explanations see sd_lavf_srt.c.
