Fix cross compilation to i686-w64-mingw32 with GCC11
Improve std::thread detection and stop assuming that the availability of std::thread means std::condition_variable will also be available.
Keeping as draft until we have a CI image using GCC11, which needs some VLC MR to be merged first as we can't cross compile using gcc11 yet.
Fix #408 (closed)
Edited by Hugo Beauzée-Luyssen