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++.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@199222 91177308-0d34-0410-b5e6-96231b3b80d8

test/testit

@@ -36,9 +36,13 @@ then
 	fi
 fi
 
+if [ -z "$CXX_LANG" ]
+then
+    CXX_LANG=c++1y
+fi
 if [ -z "$OPTIONS" ]
 then
-	OPTIONS="-std=c++11 -stdlib=libc++"
+	OPTIONS="-std=${CXX_LANG} -stdlib=libc++"
 fi
 OPTIONS="$OPTIONS -I$LIBCXX_ROOT/test/support"