preparing the buildds for the switch to GCC 5

[Matthias Klose <doko@debian.org>]

Hi,

as announced at
https://lists.debian.org/debian-devel-announce/2015/07/msg00000.html

I'm planning to do this switch on Jul 31. Fixes for the ftbfs are making
progress, and I'll try to target that date.

Instead of adding a versioned b-d for every package, I'll upload gcc-defaults
and build-essential packages with build-essential dependening on g++ (>= 4:5.1).
 Then I have to make sure that the new build-essential is installed on the
buildds. Christoph Egger suggested to contact the buildd admins to make this
change, and find some builddadm to any remaining buildds.

Matthias