I don't have a problem with your changes. Were you running into some particular problem? The thing with GNUstep Make is that it does not use the correct compiler variable for Objective-C sources. Instead of using OBJCC it uses CC to compile ObjC files (at least, this was the case the last time I checked), which is technically wrong. Additionally, in theory, as long as the same runtime was being used, CoreBase's C files could be compiled with a different compiler than Base, seeing as it talks directly to the runtime.
To comment on Lubos' comment, CoreBase does currently require an Objective-C compiler to support the ObjC -> C bridge. There are plenty of .m files in the source. Ideally, these should one day be moved to Base because they really do not belong in CoreBase. More ideally, CoreBase and Base should merge one day so that all the glue code could be simplified.