BUILD: Add `option`s to enable/disable external dependencies

[zmoon]

Currently, this would be GOCART and MUSICA-MICM.

Ah, if you want to only enable these languages only when musica is built, somewhere you can do something like this

if (ENABLE_MUSICA)
    enable_language(C)
    enable_language(CXX)
endif()

Originally posted by @K20shores in #114 (comment)