[x265] xp support

[Roger Pack]

Hello.  I noticed in a recent discussion that seemingly windows xp
support had been "dropped" to clean up code:

http://forum.videohelp.com/threads/357754-%5BHEVC%5D-x265-EXE-mingw-builds?goto=newpost

This does make it a bit painful for distributors who want to
distribute (for instance) ffmpeg with libx265 support, but also still
want to distribute "one executable" for all our users (including XP
users)...

Just throwing this out there, but maybe x265 can use pthread style
condition variables then it be compiled against win32-pthreads project
or what not, when needed.  Or something like that.
Thanks.
-roger-