Replace pthreads usage with tinycthread.

This should be helpful for Windows portability.

Signed-off-by: Evan Nemerson <evan@coeus-group.com>
windows: the core, plugins, and utils now compile with mingw.

That doesn't mean they *work*, but they do compile.  The filesystem
stuff in codec.c still doesn't work, this basically just #ifdefs all
the problematic stuff out.  Benchmark also doesn't work yet because I
need to create a Windows alternative for the timer.

Partially fixes https://github.com/quixdb/squash/issues/38

Signed-off-by: Evan Nemerson <evan@coeus-group.com>