Build option to download and build PT-SCOTCH automatically

[abishekg7]

Previously, using PT-SCOTCH for online graph partitioning with MPAS required the user to build and install it themselves and set SCOTCH= environment variable before building MPAS. While this is a completely functional option, many users may prefer the convenience of an automatic fetch and build process.

This PR adds an option to automatically fetch and build PT-SCOTCH under src/external during the MPAS build. The previously active SCOTCH variable is repurposed as a true or false build option to toggle linking PT-SCOTCH library with the MPAS build. Previously, SCOTCH pointed to the PT-SCOTCH installation path. This is now available through the SCOTCH_ROOT variable. This allows the user to still link MPAS with custom PT-SCOTCH installations.

There are a few different build + link scenarios with these two variables

1. When SCOTCH=false or unspecified, the regular MPAS build process continues without attempting to build or link against PT-SCOTCH.

make gnu CORE=init_atmosphere

2. When SCOTCH=true and SCOTCH_ROOT is not set or unset, the build process automatically fetches and builds the PT-SCOTCH graph-partitioning library, followed by the build of the MPAS model, and then the PT-SCOTCH library is linked as a static library.

make gnu CORE=init_atmosphere SCOTCH=true

3. When SCOTCH=true and SCOTCH_ROOT points to the location of an existing PT-SCOTCH installation, the regular MPAS build process completes and then the PT-SCOTCH library is linked as a static library.

export SCOTCH_ROOT=/path/to/scotch/installation
make gnu CORE=init_atmosphere SCOTCH=true

4. When SCOTCH=true and SCOTCH_ROOT is set but does not point to the location of an existing PT-SCOTCH installation, the build process exits with an error.

Notes:

• A new Externals.cfg is added under the src directory so that both the atmosphere and init_atmosphere CORES can use this external

• If the automatic PT-SCOTCH fetch and build is successful, it is installed under src/external/scotch/install. And the libraries under the lib subdirectory. However, in the case of a user-specified PT-SCOTCH installation, we search both $(SCOTCH_ROOT)/lib and $(SCOTCH_ROOT)/lib64 subdirectories to find the relevant libraries.

• SCOTCH=$(SCOTCH) is added to the options tracked by rebuild_check, so switching SCOTCH on/off between builds is detected as an incompatible-option change (triggering the clean/rebuild prompt or AUTOCLEAN).

• Add a scotch cleanup step (rm -r build install) to the clean target so make clean removes the downloaded/built SCOTCH tree along with the other external libs.

Current limitations

• CMake (>= 3.10) is presently required in order to build PT-SCOTCH using the automatic fetch + build method. In addition, all the usual dependencies of PT-SCOTCH need to be present in the environment prior to the build.

• We presently support only the linking of static PT-SCOTCH libraries with MPAS. Shared libraries are currently not supported.

[abishekg7]

Tested cases 1, 2, 3, 4 with CORE=init_atmosphere with gnu, intel and nvhpc cpu builds + runs on Derecho.