Fix checking of version number for mingw.

MinGW spits out version strings like: x86_64-w64-mingw32-g++ (GCC) 10-win32 20210110, which causes the version extraction to fail. Added support for this with tests.

Also added make_unsigned for long long since mingw seems to use that for uint64_t.

Related to #2268 (closed). CMake passes for me after this.