These variables are the ones that the standard toolchain uses, so we should use those and not always use MACOSX_DEPLOYMENT_TARGET. See https://github.com/tpoechtrager/cctools-port/blob/236a426c1205a3bfcf0dbb2e2faf2296f0a100e5/cctools/ld64/src/ld/PlatformSupport.cpp#L54-L55