Add module support and tests. (#174)

include/boost/regex/v5/object_cache.hpp

@@ -20,6 +20,7 @@
 #define BOOST_REGEX_OBJECT_CACHE_HPP
 
 #include <boost/regex/config.hpp>
+#ifndef BOOST_REGEX_AS_MODULE
 #include <memory>
 #include <map>
 #include <list>
@@ -28,6 +29,7 @@
 #ifdef BOOST_HAS_THREADS
 #include <mutex>
 #endif
+#endif
 
 namespace boost{