Honor the CXX env variable when looking for C++ compiler.

This is useful when, for instance, a build fails with gcc 4.8.
See https://code.google.com/p/v8/issues/detail?id=2149
Added --with-cxx mkmf flag.
Merge pull request #100 from jorrizza/cxx_env_variable

Honor the CXX env variable when looking for C++ compiler.