Resynchronize with pthreads4w

[dmik]

Our implementation is based on pthreads4w, formerly known as pthreads-win32. There has been a lot of changes since we last synced in 2010 (see e4528d9, 171b54d). In particular, the latest win32 implementation supports PTHREAD_MUTEX_ROBUST and other nice things.

Once this is done, #9 should be not necessary any more (and could be undone).

Note urgent (we don't have projects strongly dependent on new stuff) so postponed.