Enable modern compiler versions to use --cpp11.

The `waf configure --cpp11` flag will only enable C++11 mode when using whitelisted compilers. This updates the list of allowed compilers to include the current production release of `g++` 4.7, 4.8, 4.9 and `clang++`, since these are most commonly found on any updated system.
Wait, that won't work - it's CPP11_FLAGS, not CPP11-FLAGS - underscore, not dash