As found by @climbfuji, if you build GEOSgcm, you get three sets of
CMake files for doing find_package: MAPL, GEOSgcm, and GOCART. This then
causes circular dependencies as GOCART requires things "provided" by
GEOSgcm and GEOSgcm requires GOCART.

So this tag tries to "detect" if GOCART is being built as a standalone
project or as part of GEOS.