v3: Support for MAPL-as-library

[mathomp4]

It has been requested by some of the spack-stack folks (e.g., @climbfuji) that GEOSgcm when built with Spack should be able to use MAPL as a library rather than as a component built in.

I believe this is all that is necessary. We just need to tell CMake where mapl_stub.pl is. Because MAPL is built under ecbuild, that means it has a variable called MAPL_BASE_DIR[^1] that points to where MAPL is, e.g., in spack, where spack installed it.

[^1]: This is not great for MAPL since MAPL_Base is a thing for us, but, well, that's what ecbuild does.

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[mathomp4]

ETA: This is not enough. I'm missing something else:

[ 19%] Built target GEOSdatmodyn_GridComp
/bin/sh: generator-NOTFOUND: command not found
gmake[2]: *** [src/Components/@GEOSgcm_GridComp/GEOSagcm_GridComp/GEOSphysics_GridComp/@GEOSchem_GridComp/HEMCO_GridComp/CMakeFiles/phony_HEMCO.dir/build.make:75: src/Components/@GEOSgcm_GridComp/GEOSagcm_GridComp/GEOSphysics_GridComp/@GEOSchem_GridComp/HEMCO_GridComp/HEMCO_InternalSpec___.h] Error 127
gmake[1]: *** [CMakeFiles/Makefile2:12294: src/Components/@GEOSgcm_GridComp/GEOSagcm_GridComp/GEOSphysics_GridComp/@GEOSchem_GridComp/HEMCO_GridComp/CMakeFiles/phony_HEMCO.dir/all] Error 2

[github-actions[bot]]

This PR is being prevented from merging because you have not added one of our required labels: 0 diff, 0 diff trivial, Non 0-diff, 0 diff structural, 0-diff trivial, Not 0-diff, 0-diff, automatic, 0-diff uncoupled. Please add one so that the PR can be merged.