RFC/Ada: removal of $(ADAC) support

[Richard Kenner]

    It achieves the goal I had when I wrote it, which was to enable
    bootstrapping with no special effort on common free operating system
    distributions that ship compatible Ada and C compilers, but with the
    Ada compiler accessible only as 'gnatgcc' or similar.

Unfortunately, it didn't work and we got lots of complaints from users
of those systems.

    You rejected a patch which would have fixed this.

I don't believe it would have.

The only way to "fix" it is to use the same compiler for everything.  It
can be called either "gnatgcc" or "gcc", but the point is that you must
use the same one to compile both C and Ada.  

My objection was not that it was looking for a "gnatgcc", which is
fine, but using "gcc" to compile some files and "gnatgcc" to compile others.