...
checking callback.h usability... yes
checking callback.h presence... yes
checking for callback.h... yes
checking "for forwarding callback in runtime"... yes
checking "FFI library usage"... none
You do not have either ffcall or libffi installed, or configure needs
--with-ffi-include and/or --with-ffi-library flags so GNUstep can find
them
GNUstep requires ffcall or libffi and proper libobjc hooks to do
invocations and DO.
(This does not apply on apple-apple-apple systems where DO is
not compatible with other GNUstep systems.)