This patch takes @jroelof's idea and runs with it. However this patch doesn't require a config_site header. The patch works modifying build-libcxx/include/c++/v1/config by prepending the contents of __config_site. If extra configuration options are needed the test suite uses build-libcxx/include/c++/v1. Otherwise the test suite still uses libcxx/include.
Note that if no extra configuration options are needed then build-libcxx/include/c++/v1/__config is not modified.