[3.0] contrib: replace pthread-w32 (pthreadGC2) by winpthread

This is what we use in 4.0. It is cleaner and doesn't have the handle leak issue found in #26451 (closed).

It works on Windows XP as well, there is some conditional compilation depending if it's build with Vista support or before Vista.

Most of the patches are backported. There are a few extra patches for things that were added later in 3.0.

• backport protoc build fixes for new compilers.

Fixes #26451 (closed)

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• Added more backported patches to apply patches closer to the original ones. Incidentally, it shows the srt dependency was incorrect.
• Added the latest zvbi patches merged in master
• Documented the changes compared to the original patches.