cmake: Try winpthread before pthreadGC2

[ocrete]

It seems that pthreadGC2 is deprecated and it should prefer
winpthread. Also remove all of the commented out stuff.

The set() call seems to be superfluous with the find_library() calls. VLC had to change the "-lpthreadGC" to "-lpthread" in their build.

[ethouris]

BTW. is there any "official" pthread implementation for Windows? This Pthread-win32 for which information is attached to the repo I just found as something that can seal the hole, but if there's something more official, which doesn't need to refer to a "volatile" Github project, it would be much better.

BTW. I see a removed set and comment, but no attempt to find winpthreads. Is it this?

https://github.com/ldx/winpthreads

[ocrete]

It seems that winpthread is part of mingw64, https://sourceforge.net/p/mingw-w64/mingw-w64/ci/master/tree/mingw-w64-libraries/winpthreads/

[ethouris]

I know. I just think it would be nice to find some reliable method of having some "more reliable" pthread implementation for Windows, used not only for MinGW, but also for a case defined in the cmake as MICROSOFT. Unless Microsoft has provided already something better that I'm not aware of.