add_MARBL: only build MARBL if compset activates it

[mnlevy1981]

mnlevy1981/MOM6#21 and mnlevy1981/MOM6#22 have made it possible to build / run MOM even if MARBL is not available in pkg/MARBL provided USE_MARBL_TRACERS = False. Per mnlevy1981/MOM6#21 (comment)

An interesting thought from Matt L: if the CPP is available, we should use it in the CESM build. Meaning compsets that are not set up to run with MARBL should not build MARBL.

1. This effectively makes MARBL a build-time specification despite tracer count being run-time (but how often do CESM users decide to turn on a tracer package by hand rather than via compset?)

2. What will be the mechanism for turning MARBL on by default via compset? It seems like MOM%MARBL in the compset long name should set an env_build.xml variable - does MOM still use OCN_TRACER_MODULES or do we need something new like MOM_ENABLE_MARBL?

[mnlevy1981]

I ended up removing D_USE_MARBL_TRACERS and introduced a dummy API in config_src/externals/MARBL. There is a MOM6_BUILD_MARBL flag in env_build.xml that is TRUE by default and that indicates "build out of pkg/MARBL". The current thought process is that we want to set it to FALSE to test the dummy API, but in general will always build MARBL.