libgnatprj/libgnatvsn -dev dependencies on gnat

[Matthias Klose <doko@debian.org>]

Currently looking at https://tracker.debian.org/pkg/gcc-6-cross mot migrating, 
because the 32bit gnat targeting 64bit archs doesn't build.  There could be some 
work-arounds:

 - libgnatprj6-dev: the dependency on gnat-6 can be dropped in any
   case because libgnatprj6-dev depends on libgnatvsn6-dev, which
   depends on gnat-6 itself.

 - libgnatvsn6-dev: Drop the dependency on gnat-6 when building the
   cross compilers, keep it for the native compilers.  Of course this
   would probably break cross compilation of ada libs, but I'm not
   sure if anybody tried to do this.

 - libgnatvsn6-dev: Drop the dependency on gnat-6 for native build as well,
   and add explicit gnat build dependencies to packages currently
   build-depending on libgnatvsn6-dev.

Any preferences/ideas?  Of course it would be nice to get these cross compilers 
building again in the first place.

Matthias