base.make should go in a platform-specific directory

[Nicola Pero]

base.make contains the flags used to compile against gnustep-base on that
platform.

It is currently installed into GNUSTEP_MAKEFILES/Additional/.  But if the
system is non-flattened, should we have a platform-dependent directory in
there, and install base.make in that directory, as it contains
platform-specific flags ?

I think nobody is using this functionality, so it probably makes not much 
difference, but anyway.