Coreneuron doesn't build properly with AppleClang and libomp (via brew) on OSX

[pramodk]

Describe the issue

If we install OpenMP runtime via brew and try to build CoreNEURON with AppleClang then building as well as running tests fail.

To Reproduce

brew install libomp

# cd to build dir
cmake ..
make -j

Consolidate compiler generated dependencies of target corenrn_mpi
[ 31%] Building CXX object coreneuron/CMakeFiles/corenrn_mpi.dir/mpi/lib/nrnmpi.cpp.o
/Users/kumbhar/workarena/repos/bbp/coreneuron/coreneuron/mpi/lib/nrnmpi.cpp:18:10: fatal error: 'omp.h' file not found
#include <omp.h>
         ^~~~~~~

Expected behavior

Build should be successful with or without libomp.

System (please complete the following information)

• OS: Mac OS
• Compiler: AppleClang

Additional context

• See the BBP slack discussion
• AppleClang never ships OpenMP. Recently MacOS CI images on Azure & GitHub are coming with libomp and that's when we are seeing the issue