[Deploy] Enable OpenMP for Apple Clang

[phcerdan]

CMake Error at cmake-3.18/Modules/FindPackageHandleStandardArgs.cmake:165 (message):
  Could NOT find OpenMP_C (missing: OpenMP_C_FLAGS OpenMP_C_LIB_NAMES)

It seems that the compiler AppleClang 11.0.3.11030032 does not ship openmp?

Read more on:
https://iscinumpy.gitlab.io/post/omp-on-high-sierra/

Meanwhile, openmp in DGtal when WITH_OPENMP:BOOL=ON cannot be enabled.