Bug#760343: transition: protobuf 2.6.0

[Robert Edmonds <edmonds@debian.org>]

I see a build failure on mips in the 'shogun' package:

    https://buildd.debian.org/status/fetch.php?pkg=shogun&arch=mips&ver=3.2.0-7.2%2Bb1&stamp=1410641206

This is actually an architecture+compiler specific build failure.
(Funnily enough, shogun uses "clang++" on some architectures but "c++"
on others as the compiler.)  The problem is that the 'generic' atomic
implementation fallback is only used when the compiler is actually
gcc/g++, rather than clang/clang++.

The fix is relatively simple (basically we just need to also detect
clang as well as gcc >= 4.7), but it has to be done in the protobuf
headers, which will mean another protobuf upload.

-- 
Robert Edmonds
edmonds@debian.org