RFC/Ada: removal of $(ADAC) support

[Robert Dewar]

> You rejected a patch which would have fixed this.


You fixed one particular aspect, but there is no guarantee that changes made
to GNAT will be compatible with various ill-matched GNAT/C compilers, we don't
test for that, and indeed how can you test over some ill-defined universe.

I still don't understand why you would want to make additional difficulties
here.