Fix pthread.h detection in rig.h

We may have to start distributing config.h with hamlib
https://github.com/Hamlib/Hamlib/issues/947

include/hamlib/rig.h

@@ -2552,12 +2552,12 @@ struct rig_state {
     void *async_data_handler_priv_data;
     volatile int poll_routine_thread_run;
     void *poll_routine_priv_data;
-#ifdef HAVE_PTHREAD
-    pthread_mutex_t mutex_set_transaction;
-#endif
     hamlib_port_t rigport;  /*!< Rig port (internal use). */
     hamlib_port_t pttport;  /*!< PTT port (internal use). */
     hamlib_port_t dcdport;  /*!< DCD port (internal use). */
+#ifdef _PTHREAD_H
+    pthread_mutex_t mutex_set_transaction;
+#endif
     /********* DO NOT ADD or CHANGE anything (or than to rename) ABOVE THIS LINE *********/
     /********* ENSURE ANY NEW ITEMS ARE ADDED AFTER HERE *********/
     /* flags instructing the rig_get routines to use cached values when asyncio is in use */