Specifying which libraries to use is done with
./configure --with-library-combo=objcruntime-foundation-applicationkit. Each of the three could have either the value gnu or apple, but not all of these are implemented.
To use the GNUstep runtime for ObjectiveC-2 with all the latest cutting edge changes, you should specify the 'ng' (Next Generation) runtime library (which also implies use of a recent clang compiler)
The following options are supported:
Of these, the first two, and the last option are recommended (all GNU or all Apple). The default when building on Darwin and OS X is apple-apple-apple. Even when GNUstep apps use the all Apple option, GNUstep extensions are available to the app.