Compile MPAS with CMake

[mark-petersen]

There are currently CMake files for each core, in order to compile E3SM with CMake, added in #386. The next step is to add the CMake files so that we can compile MPAS standalone, i.e. replace a command like

make gfortran CORE=ocean OPENMP=true DEBUG=true GEN_F90=true

with the corresponding CMake command. This issue provides a place to discuss that work.

[mark-petersen]

@jgfouca could you outline what additional files are needed to compile MPAS stand-alone with CMake? We are new to CMake, so pointing to similar examples is also helpful.

[mark-petersen]

The current Makefile has many sections like this:

MPAS-Model/Makefile

Lines 326 to 336 in 51d5624

	gnu-nersc:
	( $(MAKE) all \
	"FC_PARALLEL = ftn" \
	"CC_PARALLEL = cc" \
	"CXX_PARALLEL = CC" \
	"FC_SERIAL = ftn" \
	"CC_SERIAL = cc" \
	"CXX_SERIAL = CC" \
	"FFLAGS_PROMOTION = -fdefault-real-8 -fdefault-double-8" \
	"FFLAGS_OPT = -O3 -m64 -ffree-line-length-none -fconvert=big-endian -ffree-form" \
	"CFLAGS_OPT = -O3 -m64" \

that specify details on a machine and compiler-specific level. Does CMake auto-detect most of that, but still allow us to specify flags when needed?

[jgfouca]

@mark-petersen , the main missing pieces from what I can remember are machine files and a general flag config file.

For example, your snippet above would become a gnu-nersc.cmake file containing:

set(CMAKE_Fortran_COMPILER ftn)
set(CMAKE_C_COMPILER cc)
set(CMAKE_CXX_COMPILER CC)
set(FFLAGS_PROMOTION  "-fdefault-real-8 -fdefault-double-8")
set(CMAKE_Fortran_FLAGS_RELEASE "-O3 -m64 -ffree-line-length-none -fconvert=big-endian -ffree-form")
set(CMAKE_C_FLAGS_RELEASE "-O3 -m64")