Previously benchmark_api.h wasn't allowed to include standard library
headers. For this reason SetLabel had a hack to accept std::string
without including <string>. The hack worked by attempting to detect
the injected class name `basic_string`. However Clang has changed
it's behavior regarding injected class names so this hack no longer
works.
This patch removes the hack and replaces it with a function that
actually names std::string. However we still cannot pass std::string
across the dylib boundary because of libstdc++'s dual C++11 ABI.
