RFC/Ada: removal of $(ADAC) support

[Richard Kenner]

    I don't want to have the huge argument again that we had when this
    went in, but I'll reiterate what I said at the time: This mechanism
    should be removed no earlier than when there is a released stable
    version of Debian GNU/Linux for which GCC 3.2 or later is the default
    compiler.  Realistically speaking, that means sometime in mid-2004.
    That is also the earliest time at which features of GNU Ada present
    only in GCC 3.2 can be used in GNAT.

But the problem is that the $(ADAC) mechanism basically *doesn't
work*.  In general, you can't compile the Ada sources with a different
version of GCC from the C files due to the potential for incompatible
exception handling.  Nearly all of the build problems with Ada have been
due to this.