Fix OpenMP support for MPAS standalone builds with cray-cray

[xylar]

A -fopenmp (rather than a -homp) flag seems to be needed for OpenMP on Frontier.

For MPAS component standalone builds only -- does not impact E3SM

[BFB]

[xylar]

Testing

I'm able to build standalone MPAS-Ocean with make cray-cray OPENMP=true with this branch on Frontier, whereas I see:

...
Testing compiler for OpenMP support
clang-15: warning: argument unused during compilation: '-h noacc' [-Wunused-command-line-argument]
clang-15: warning: argument unused during compilation: '-h omp' [-Wunused-command-line-argument]
ld.lld: error: undefined symbol: omp_get_num_threads
>>> referenced by conftest.c
>>>               /tmp/conftest-f9b2d4.o:(main)
clang-15: error: linker command failed with exit code 1 (use -v to see invocation)
cc does not support OpenMP - see INSTALL in top-level directory for more information
make[1]: *** [Makefile:939: openmp_test] Error 1
make[1]: Leaving directory '/autofs/nccs-svm1_home1/xylar/e3sm_work/polaris/add-omega-ctest-util/e3sm_submodules/crayclang/components/mpas-ocean'
make: *** [Makefile:444: cray-cray] Error 2

without these changes.

I was also able to run the pr suite from Polaris using MPAS-Ocean build with this branch. Most tests ran successfully and the failure in inertial_gravity_wave is unrelated to this fix. (I have made a fix to Polaris to the parallel config options for Frontier to use one thread per core.)

[philipwjones]

I have reproduced the bug and verified that this change does indeed fix the build.

[xylar]

Thanks @philipwjones! That was fast!

[jonbob]

passes:

• ERS.ne30pg2_r05_EC30to60E2r2.GPMPAS-JRA.chrysalis_intel.mosart-rof_ocn_2way
• SMS_D_Ld3.T62_oQU120.CMPASO-IAF.chrysalis_intel

merged to next

[jonbob]

merged to master