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

Addresses issue https://github.com/SuperElastix/elastix/issues/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 https://github.com/openMVG/openMVG/pull/2038 "builds on Mac OS 12 AppleClang + OpenMP"

SuperElastix/elastix,SuperElastix/elastix,SuperElastix/elastix,SuperElastix/elastix,SuperElastix/elastix