Bug#780640: gcc-5: merge gnat back to gcc

[Matthias Klose <doko@debian.org>]

some comments:

 - please add appropriate changelog entries
 - don't rely on autogen during the build. I was just
   happy to get rid off it. I think it's fine
   to keep the auto generated toplevel Makefile.
   It doesn't change that often.

I think it makes sense to build gnat out of the gcc-5 source package. I talked
to Ludovic about this at Fosdem, and he didn't have major concerns.  And
probably he'll update Ada only once during the next release cycle, and that
likely will be for GCC 6.