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

I haven't tested it yet, because the latest master fails to build with `make: *** No rule to make target 'x64.release'.  Stop.` on my Debian Sid box. Since this change practically can't break, I decided to send in the pull request anyway.