qt6-base: improve clang32 non-CI parallelism check
qt6-base: improve clang32 non-CI parallelism check

Don't override CMAKE_BUILD_PARALLEL_LEVEL to 4 if
CMAKE_BUILD_PARALLEL_LEVEL is already defined (assume the user knows
what they're doing), or if there are fewer than 4 processors (so the
default may be lower than 4).

fabiangreffrath/MINGW-packages,oroppas/MINGW-packages,oroppas/MINGW-packages,mati865/MINGW-packages,InitialForce/MINGW-packages,oscarfv/MINGW-packages,lhmouse/MINGW-packages,oscarfv/MINGW-packages,jtanx/MINGW-packages,Ede123/MINGW-packages,Alexpux/MINGW-packages,mati865/MINGW-packages,lhmouse/MINGW-packages,InitialForce/MINGW-packages,Alexpux/MINGW-packages,oroppas/MINGW-packages,JPeterMugaas/MINGW-packages,fabiangreffrath/MINGW-packages,mati865/MINGW-packages,Optiligence/MINGW-packages,mati865/MINGW-packages,stahta01/MINGW-packages,oscarfv/MINGW-packages,JPeterMugaas/MINGW-packages,oscarfv/MINGW-packages,Alexpux/MINGW-packages,InitialForce/MINGW-packages,JPeterMugaas/MINGW-packages,jtanx/MINGW-packages,fabiangreffrath/MINGW-packages,Optiligence/MINGW-packages,fabiangreffrath/MINGW-packages,stahta01/MINGW-packages,oroppas/MINGW-packages,Chocobo1/MINGW-packages,Alexpux/MINGW-packages,Ede123/MINGW-packages,fabiangreffrath/MINGW-packages,Optiligence/MINGW-packages,stahta01/MINGW-packages,oscarfv/MINGW-packages,Ede123/MINGW-packages,Chocobo1/MINGW-packages,jtanx/MINGW-packages,lhmouse/MINGW-packages,lhmouse/MINGW-packages,lhmouse/MINGW-packages,stahta01/MINGW-packages,JPeterMugaas/MINGW-packages,Ede123/MINGW-packages,JPeterMugaas/MINGW-packages,JPeterMugaas/MINGW-packages,oscarfv/MINGW-packages,mati865/MINGW-packages,fabiangreffrath/MINGW-packages,Chocobo1/MINGW-packages,Alexpux/MINGW-packages,stahta01/MINGW-packages,lhmouse/MINGW-packages,oroppas/MINGW-packages,Optiligence/MINGW-packages,stahta01/MINGW-packages,oroppas/MINGW-packages,jtanx/MINGW-packages,Alexpux/MINGW-packages