GCC 4.6 compatibility

[mlange05]

Ubuntu LTS and travis use GCC 4.6 as the default compiler, so we should aim to support it. We currently don't due to the use of non-static data member initializers, which is a C++11 feature and easy to change.

[marcoscimatec]

Shall we change it after the cgen code is merged?

[mlange05]

Yes, lets land PR #60 first. It will make this one easier, I think.