enforce GCC5 C++11 ABI and usage

The library(s) make an API break anyhow, so lets ensure we use gcc5 for
this break and enable c++11 as standard as gcc6 will use it as default
and should provide some API parts for c++11 – beside that it can't hurt
to use c++11 itself. We just have to keep our headers c++03 compatible
to not enforce a standrd bump in our reverse dependencies.