RFC/Ada: removal of $(ADAC) support

[Richard Kenner]

    On some popular architectures it can.  Debian did experience a number
    of bootstrap failures on other architectures when we frst transitioned
    to GCC 3.2 however.  It was also particularly sensitive to small
    perturbations in the compiler being bootstrapped, making it
    particularly frustrating.

    Sorry, I no longer have the logs, so consider this only informed hearsay.

Do you remember which architectures?  Were they ones on which GNAT is
supported?