gcc-7 migration

[Nicolas Boulenguez <nicolas@debian.org>]

Hello.

In testing and unstable, most Ada packages FTBFS because some changes
libgnat6-dev invalidate their .ali files. Fixing this requires a
source upload and a passage through the NEW queue for every library.

[1] https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=866355

In experimental, the migration to gnat-7 is on its way.  Most FTBFS
are also caused by obsolete .ali files, but it is sufficient to
recompile as the ALI/SO versions are already diffenent *from the ones
in unstable*.

asis aunit dh-ada-library florist gmpada gnatcoll gpr gtkada
ncursesada opentoken templates-parser texttools xmlada xmlezout
are mostly completed.

adabrowse adacgi adacontrol adasockets ahven alog anet aws dbusada
gnat-gps liblog4ada pcscada polyorb spark topal
need to be updated.

Now that gcc-7 is available in testing, and its ali files do not
change anymore, I suggest that we start uploading to unstable.