PyGILState_Ensure(), PyGILState_Release(), PyGILState_GetThisThreadState() are

not available if Python is compiled without threads.

Include/pystate.h

@@ -158,6 +158,8 @@ typedef
     enum {PyGILState_LOCKED, PyGILState_UNLOCKED}
         PyGILState_STATE;
 
+#ifdef WITH_THREAD
+
 /* Ensure that the current thread is ready to call the Python
    C API, regardless of the current state of Python, or of its
    thread lock.  This may be called as many times as desired
@@ -199,6 +201,8 @@ PyAPI_FUNC(void) PyGILState_Release(PyGILState_STATE);
 */
 PyAPI_FUNC(PyThreadState *) PyGILState_GetThisThreadState(void);
 
+#endif   /* #ifdef WITH_THREAD */
+
 /* The implementation of sys._current_frames()  Returns a dict mapping
    thread id to that thread's current frame.
 */