Specify the C++ variant/dialect used by Caffe

[eelstork]

Currently the C++ version used by Caffe is left unspecified. This may hinder cross platform support and confuse Caffe contributors.
De facto, Caffe is tested against C++98 with GNU extensions (via GCC 4.6.3 on Travis).
I understand that C++11 is fairly well supported on most platforms; I believe that it is too early to designate C++11 as the minimum version (see #2537).
To improve compatibliity I suggest that we disable GNU extensions support.

[bhack]

We have a more restrictive policy on cudnn and cuda that on GCC. GCC 4.8 it is enough to compile with c++11 without boost. Only filesystem like boost features will be available in more advanced GCC CLANG versions.