Update for working with MPI from NCEPLIBS-external or elsewhere

[climbfuji]

Update CMakeLists.txt:

• can use MPI from NCEPLIBS-external or elsewhere
• bugfix for WGRIB2 (need to set variable WGRIB2_ROOT)
• additional variables written to the shell scripts that set the environment variables for compiling the UFS model

Also update .gitmodules to change the branch name for NCEPLIBS-post (aka EMC_post) to release/public-v4, see NOAA-EMC/UPP#64.

[climbfuji]

I tested this PR together with NOAA-EMC/NCEPLIBS-external#12 successfully for two different setups:

• using gcc+gfortran with no MPI library installed, i.e. install it as part of NCEPLIBS-external
• using clang+gfortran with MPI library installed, i.e. add -DBUILD_MPI=OFF to the NCEPLIBS-external build

[climbfuji]

This PR must be merged together with NOAA-EMC/NCEPLIBS-external#12