Disable physics for MPAS dycore-only build

[kuanchihwang]

When building MPAS as a dynamical core, all physics-related components are disabled. This build configuration is usually used by CAM/CAM-SIMA, and can be achieved by defining the MPAS_CAM_DYCORE macro in CPPFLAGS.

On the other hand, the PHYSICS Makefile variable controls whether physics are enabled in MPAS, but its logic is decoupled from the MPAS_CAM_DYCORE macro. Disabling physics in MPAS currently requires manual interventions.

This PR disables physics automatically when the MPAS_CAM_DYCORE macro is found in CPPFLAGS. It mainly benefits CAM-SIMA, which reuses the build infrastructure of MPAS for building it as a dynamical core.

For stand-alone MPAS and CAM, this PR is an NFC (No Functional Change).

[abishekg7]

Is the standalone MPAS build supposed to work if you add MPAS_CAM_DYCORE or -DMPAS_CAM_DYCORE to the CPPFLAGS in the root directory's Makefile?

[kuanchihwang]

@abishekg7

Not really. This build configuration is usually used by CAM(-SIMA), where MPAS will be compiled as a part of another host model. When the MPAS_CAM_DYCORE macro is defined, it introduces an external dependency, which comes from the host model. Therefore, MPAS cannot be compiled independently under this build configuration.

To review this PR, I would suggest the following on Derecho:

git clone https://github.com/ESCOMP/CAM-SIMA.git
cd CAM-SIMA
git checkout development
./bin/git-fleximod update
mkdir -pv ../PR1281 && cd ../PR1281

# Compile with GNU compilers
../CAM-SIMA/cime/scripts/create_newcase --compiler gnu --case PR1281-GNU --compset FKESSLER --project PROJECT_KEY --res mpasa480_mpasa480 --run-unsupported
cd PR1281-GNU
# Compile with Intel compilers
../CAM-SIMA/cime/scripts/create_newcase --compiler intel --case PR1281-Intel --compset FKESSLER --project PROJECT_KEY --res mpasa480_mpasa480 --run-unsupported
cd PR1281-Intel

./case.setup
./case.build

# Inspect the build artifacts of MPAS at:
#   "$SCRATCH/PR1281-GNU/bld/atm/obj/mpas" for GNU compilers
#   "$SCRATCH/PR1281-Intel/bld/atm/obj/mpas" for Intel compilers

The changes that you are reviewing have been applied as a patch: 0002-Disable-physics-for-MPAS-dycore-only-build.patch. This PR is an attempt to upstream that particular patch so we do not have to maintain it downstream at CAM-SIMA.

[abishekg7]

I've tried building standalone MPAS - seems fine. And I also followed @kuanchihwang 's instructions for CAM-SIMA. Also seems to build fine.