Better fix for MinGW compilation
This supercedes !2 (merged) with a better approach. As both CMAKE_USE_WIN32_THREADS_INIT
and CMAKE_USE_PTHREADS_INIT
are defined when using MinGW, check for the first one before checking for the second.
@ben.boeckel COuld this be merged also in VTK 8.2 or is too late?