Use OpenMP_CXX_INCLUDE_DIRS, fix OpenMP_CXX_FLAGS usage
These are fixes for issues encountered building VTK-m 1.6.0 with AppleClang 1184.108.40.206 (Xcode 12.4 command line tools) on macOS 10.15.7 with an external OpenMP runtime (libomp 13 from MacPorts).
OpenMP_CXX_INCLUDE_DIRSif set: Compilers such as AppleClang do not include OpenMP headers. Specifying the path to OpenMP headers using
OpenMP_CXX_INCLUDE_DIRduring configure may cause the
OpenMP_CXX_INCLUDE_DIRSvariable to be set (see https://cmake.org/cmake/help/latest/module/FindOpenMP.html), which if not used during the build will cause fatal errors due to omp.h not being found.
OpenMP_CXX_FLAGSfrom space-separated list to semicolon-separated list before appending to compiler options in order to avoid warnings from AppleClang during build caused by
-Xclang -fopenmpbeing treated as one argument rather than two:
clang: warning: argument unused during compilation: '-Xclang -fopenmp' [-Wunused-command-line-argument]