Bug#903694: bootstrap issue

[Nicolas Boulenguez <nicolas@debian.org>]

gnat-7/7.3.0-26 wrongly assumes that /usr/bin/gcc links to gcc-7.
By chance, this currently holds in unstable,
and gcc-7 is hopefully able to build a fixed version of itself.

gnat-8/8.1.0-11 wrongly assumes that /usr/bin/gcc links to gcc-8.
It is broken in unstable, at least with default options.

So the fresh gcc-8/8.1.0-12 fails to build with gcc-8/8.1.0-11.

I suggest that
  gcc-8/8.1.0-13 Build-Depends: gcc-7
again until 8.1.0-13 is available on all architectures, then
  gcc-8/8.1.0-14 Build-Depends: gcc-8 (>= 8.1.0-13)

Another option is to append --GCC=gcc-8 --GNATLINK=gnatlink-8 etc to
each /usr/bin/gnatmake invokation.  A second upload would also be
necessary once the fixed gnat-8 is available, in order to remove the
options, because such work-arounds have repeatedly resulted in broken
crosss-builds or bootstraps.