Bug #15327: configure: --with-tcp-port option being partially ignored

make sure that if builder configured with a non-standard (!= 3306)
default TCP port that value actually gets used throughout. if they
didn't configure a value, assume "use a sensible default", which
will be read from /etc/services or, failing that, from the factory
default. That makes the order of preference
- command-line option
- my.cnf, where applicable
- $MYSQL_TCP_PORT environment variable
- /etc/services (unless configured --with-tcp-port)
- default port (--with-tcp-port=... or factory default)

libmysql/libmysql.c

@@ -133,10 +133,23 @@ int STDCALL mysql_server_init(int argc __attribute__((unused)),
       {
 	struct servent *serv_ptr;
 	char	*env;
-	if ((serv_ptr = getservbyname("mysql", "tcp")))
-	  mysql_port = (uint) ntohs((ushort) serv_ptr->s_port);
-	if ((env = getenv("MYSQL_TCP_PORT")))
-	  mysql_port =(uint) atoi(env);
+
+        /*
+          if builder specifically requested a default port, use that
+          (even if it coincides with our factory default).
+          only if they didn't do we check /etc/services (and, failing
+          on that, fall back to the factory default of 3306).
+          either default can be overridden by the environment variable
+          MYSQL_TCP_PORT, which in turn can be overridden with command
+          line options.
+        */
+
+#if MYSQL_PORT_DEFAULT == 0
+        if ((serv_ptr = getservbyname("mysql", "tcp")))
+          mysql_port = (uint) ntohs((ushort) serv_ptr->s_port);
+#endif
+        if ((env = getenv("MYSQL_TCP_PORT")))
+          mysql_port =(uint) atoi(env);
       }
 #endif
     }