Add native threading support on Windows.
Add native threading support on Windows.

Now liblzma only uses "mythread" functions and types
which are defined in mythread.h matching the desired
threading method.

Before Windows Vista, there is no direct equivalent to
pthread condition variables. Since this package doesn't
use pthread_cond_broadcast(), pre-Vista threading can
still be kept quite simple. The pre-Vista code doesn't
use anything that wasn't already available in Windows 95,
so the binaries should run even on Windows 95 if someone
happens to care.

pombreda/go-liblzma,pombreda/go-liblzma,pombreda/go-liblzma