Updates for Spack Stack

[mathomp4]

This PR is needed for building GEOSgcm with spack-stack (see JCSDA/spack#174).

Here we update the main CMakeLists.txt for the case where Baselibs is not found so that all the needed find_package() and add_library(ALIAS) calls can be made. Testing shows that this is zero-diff.

I've also added a test for the esmf_ROOT environment variable that Spack creates and use that to find the FindESMF.cmake file. Not perfect, but now when building by hand you don't have to do -DCMAKE_MODULE_PATH=${esmf_ROOT}/cmake.

Also, this adds a smidge of the changes needed for FMS-in-spack. I can't turn this on by default due to changes that are needed in fvdycore (see GEOS-ESM/GFDL_atmos_cubed_sphere#51). As this is an interface change, we have to go whole-hog into it.

Related PRs that are NOT necessary, just future proofing for FMS-in-Baselibs/FMS-in-spack:

• Updates for Spack Stack FVdycoreCubed_GridComp#180 → Released as v1.12.0
• Updates for Spack Stack GFDL_atmos_cubed_sphere#54 → Released as geos/v1.5.0
• Updates for Spack Stack GEOSgcm_GridComp#643 → Currently in develop

(And there is also GEOS-ESM/ESMA_env#82 but this is going to be a ways away...)