RFC/Ada: removal of $(ADAC)

[Richard Kenner]

    I'm not up to speed on the arguments for why this isn't the default.

Basically because no one has done it yet and that's because nobody has
figured out how to do it.  I suspect part of that task is going to be the
splitting up of the files in the ada directory into those for the compiler
and those for the library, but it's tricky because the compiler needs to
use the library files when it's built.