Update lib/buildit and test/testit to both pay attention to an env flag...

Update lib/buildit and test/testit to both pay attention to an env flag CXX_LANG, which people can set to 'c++03', 'c++11' or 'c++1y' to build/test using that language variant. If you don't set this env variable, you get c++11, just like today. Drive-by fix; remove duplicate -nostdinc++.

llvm-svn: 199222