Add SharedEnvExtension#gcc_with_cxx11_support? to centralise the
logic for checking whether a compiler is known to support C++11.
Update logic to accept GCC 4.8 and above (including 6). Thereby also
address oversight in #163 where support for GCC 6 was added without
updating the C++11 compiler whitelist.
Add tests for Superenv#cxx11.
Closes#346.
01e8e180a8