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.