COMP: Fix MacOS CI errors, "‘omp.h’ file not found", "Undefined symbols"

Addresses issue #650 "MacOS GitHub Actions, Azure Pipelines fatal error: ‘omp.h’ file not found"

Also fixed MacOS/OpenMP linker errors, saying:

> Undefined symbols for architecture x86_64:
> "___kmpc_for_static_fini", referenced from:
>     _.omp_outlined. in itkAdvanceOneStepParallellizationTest.cxx.o
>  "___kmpc_for_static_init_4", referenced from:
>     _.omp_outlined. in itkAdvanceOneStepParallellizationTest.cxx.o
>  "___kmpc_fork_call", referenced from:
>      OptimizerTEMP::AdvanceOneStep() in itkAdvanceOneStepParallellizationTest.cxx.o
>  "_omp_set_num_threads", referenced from:
>      OptimizerTEMP::AdvanceOneStep() in itkAdvanceOneStepParallellizationTest.cxx.o

Inspired by pull request openMVG/openMVG#2038 "builds on Mac OS 12 AppleClang + OpenMP"

CMakeLists.txt

@@ -125,6 +125,8 @@ if(ELASTIX_USE_OPENMP)
     set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} ${OpenMP_CXX_FLAGS}")
     #  set(CMAKE_EXE_LINKER_FLAGS "${CMAKE_EXE_LINKER_FLAGS}
     #  ${OpenMP_EXE_LINKER_FLAGS}")
+    include_directories(${OpenMP_CXX_INCLUDE_DIRS})
+    link_libraries(${OpenMP_CXX_LIBRARIES})
     add_definitions(-DELASTIX_USE_OPENMP)
   endif()
 endif()