configure: use gcc and g++ as CC and CXX defaults

It matches what GYP's Makefile generator does and it should improve
compiler detection because cc and c++ are not always gcc and g++.

Fixes: https://github.com/iojs/io.js/issues/1173
PR-URL: https://github.com/iojs/io.js/pull/1174


Reviewed-By: Johan Bergström <bugs@bergstroem.nu>