New search path when compiling MOM6

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The build instructions for MOM6 have recently changed. The search path for source code must now include config_src/external/. The build instructions on the wiki have been updated. Please update your build scripts if you sync with dev/gfdl after today and include commit NOAA-GFDL/MOM6@c2f6f77.

The purpose of the config_src/external directory is to contain null versions (just the interfaces) of external packages that we may optionally build with. This will allow us to avoid an expanding network of submodules moving forward and also provides a small degree of testing with those interfaces. Only one external package exists so far (GFDL's ocean_BGC) but more will be added shortly. If you wish to build with the actual package then you should swap in the appropriate path for the package in place of the null interface.