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.
Just so I can move this file without modifying its contents in the next
commit.
compat/compiler.h is to be moved to osdep/ with the next commit, so add
a dummy header.