This fixes the build in mingw-w64/Clang on MSYS2. It also disables the
use of gnu_printf in Clang, which was what was causing most of the
warnings. The Clang-compiled mpv binary appears to work, but there are
no guarantees yet, since until now mpv has only been tested with
mingw-w64/GCC on Windows.
Fixes#3800
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.
