Update on GPRbuild and the new GCC

[Björn Persson]

GPRbuild has been re-bootstrapped on ARM, so there is now a statically
linked GPRbuild in Rawhide on all three primary architectures. It seems
to work well, because it can build other packages without errors. With
this in place we'll be able to rebuild packages after future GCC
upgrades.

The following packages have been rebuilt with GCC 6 and linked to the
new libgnat:

ahven
anet
aws
florist
gnatcoll
gprbuild
GtkAda
GtkAda3
mine_detector
PragmARC
templates_parser
xmlada
zlib-ada

A scratch build of Matreshka succeeded and the official build is now
running.

Björn