Sync configure.ac with GCC

[Kaveh R. Ghazi]

At the behest of Paolo Bonzini, I've copied over two recent patches I made 
to the top-level configure.ac from GCC:

http://gcc.gnu.org/ml/gcc-patches/2009-11/msg00297.html
http://gcc.gnu.org/ml/gcc-patches/2009-11/msg00731.html

These patches should have no visible effect if a gcc/ directory does not 
exist.  But if I inadvertently broke something, I'm sure you'll let me know.

        Regards,
        --Kaveh
--
Kaveh R. Ghazi