FindOpenMP.cmake doesn't create OpenMP_<lang>_VERSION variables as cache variables
Unlike the OpenMP_<lang>_SPEC_DATE
variable, the OpenMP_<lang>_VERSION
, OpenMP_<lang>_VERSION_MAJOR
and OpenMP_<lang>_VERSION_MINOR
variables are not cache variables. This inconsistency should be documented if it is intended, or if it is unintended the fix is simply to make them cache variables.