Use OpenMP_CXX_INCLUDE_DIRS, fix OpenMP_CXX_FLAGS usage

These are fixes for issues encountered building VTK-m 1.6.0 with AppleClang 1200.0.32.29 (Xcode 12.4 command line tools) on macOS 10.15.7 with an external OpenMP runtime (libomp 13 from MacPorts).

• Use OpenMP_CXX_INCLUDE_DIRS if set: Compilers such as AppleClang do not include OpenMP headers. Specifying the path to OpenMP headers using OpenMP_CXX_INCLUDE_DIR during configure may cause the OpenMP_CXX_INCLUDE_DIRS variable 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.

• Convert OpenMP_CXX_FLAGS from space-separated list to semicolon-separated list before appending to compiler options in order to avoid warnings from AppleClang during build caused by -Xclang -fopenmp being treated as one argument rather than two:

clang: warning: argument unused during compilation: '-Xclang -fopenmp' [-Wunused-command-line-argument]