gnat-6 transition

[Nicolas Boulenguez <nicolas@debian.org>]

Hello.

Now that gnat Depends: gnat-6, and to the extent of my knowledge, all
Ada packages
- either have migrated to gnat-6
- or should only require a recompilation request:

  spark

- or fail to build because they Build-Depend: gnat, gnat-4.9:

  adabrowse
  adacgi
  adasockets
  ahven
  anet
  apq
  dbusada
  gnat-gps      (indirectly via versioned -dev packages)
  libalog
  libaws
  libflorist
  liblog4ada
  libncursesada
  libxmlezout   (should Build-Depend: gnat, gnat-X per policy)
  pcscada
  polyorb       (should Build-Depend: gnat, gnat-X per policy)
  topal

I intend to fill a bug against theses packages, so that no one spends
time investigating separate FTBFS on build daemons.

For libraries, depending on a new compiler implies a change of ALI and
SO versions, then an upload to the NEW queue. This is a good
opportunity to package a new upstream version if one is available, or
to drop any manual -dbg package.

Depending on the package, it may be of interest that
- gprbuild requires a version >= 2015-2 for gnat-6.
- dh-ada-library requires a version >= 6.7 for  gnat-6.
- /u/s/ada/debian-packaging.mk is now versioned and should be
  included as /u/s/ada/debian-packaging*.mk.
- currently gcc=gcc-5, so builds mixing languages should be checked in
  logs. /u/s/doc/gprbuild/README or /u/s/ada/debian-packaging*.mk may
  be useful.
- the NEW queue refuses source-only .changes files.