gcc has a pb because the template mesh<Variable,Opt>
has the name same as the gf mesh method (!).
Clang is fine however on this...
Solution : rename the template mesh<...> to gf_mesh...
Not very elegant, but ok.
new policy : for non standard conforming compilers,
we can skip the tests using new features using recent c++ features.
I.e. compilers below gcc 4.8.1, clang 3.3.