gnustep-make tweaks for latest compiler/runtime features

[Richard Frith-Macdonald]

Today I committed changes (mostly to make, but a few to base as well) to 
improve the ng-gnu-gnu library combo support for using the latest 
compiler/runtime features.

Now when you specify the 'ng' runtime, the make configure script should 
automatically try to use clang rather than gcc (unless you explicitly set the 
environment variables to say otherwise).
You do still need to have the correct runtime installed in a standard location 
so that configure will find it before any other runtime (or you need to supply 
flags to make sure that configure finds it first), but apart from that it 
should be sufficient to do 'configure --with-library-combo=ng-gnu-gnu' to build 
everything using all the latest features,

I also modified gnustep-make so that you can define GS_WITH_ARC=1 (at the start 
of your makefile, or in the environment, or on the make command line) to have a 
project built using ARC;  hopefully that will make experimenting with ARC much 
easier.

Please give it a try so we can iron out any bugs/problems quickly.