only use /openmp if MSVC is able to compile src/test/has_openmp.cpp
only use /openmp if MSVC is able to compile src/test/has_openmp.cpp

fredwang00/primesieve,tempbottle/primesieve,kimwalisch/primesieve,anatoliyrazin/primesieve,ZahidDev/primesieve,anatoliyrazin/primesieve,ZahidDev/primesieve,tempbottle/primesieve,anatoliyrazin/primesieve,fredwang00/primesieve,kimwalisch/primesieve,tempbottle/primesieve,kimwalisch/primesieve,ZahidDev/primesieve,fredwang00/primesieve