remove more __MINGW32__ ifdefs

[oltolm]

This PR removes more __MINGW32__ ifdefs and fixes some CMake warnings. CMake shows this warning:

[cmake] CMake Deprecation Warning at CMakeLists.txt:2 (cmake_minimum_required):
[cmake]   Compatibility with CMake < 3.10 will be removed from a future version of
[cmake]   CMake.
[cmake] 
[cmake]   Update the VERSION argument <min> value.  Or, use the <min>...<max> syntax
[cmake]   to tell CMake that the project requires at least <min> but has been updated
[cmake]   to work with policies introduced by <max> or earlier.

Maybe we should just remove cmake_minimum_required from all CMakeLists.txt but the top one.

[hrydgard]

Yeah, probably sensible to clean out those minimum_required.

Great cleanups!