Fix mosquitto_loop_stop() not stopping on Windows

Closes #1748. Closes #117. Thanks to Sigmund Vik.

ChangeLog.txt

@@ -27,6 +27,7 @@ Client library:
   connected. Closes #1725.
 - `mosquitto_loop_start()` now sets a thread name on Linux, FreeBSD, NetBSD,
   and OpenBSD. Closes #1777.
+- Fix `mosquitto_loop_stop()` not stopping on Windows. Closes #1748. Closes #117.
 
 
 1.6.10 - 2020-05-25

lib/dummypthread.h

@@ -4,6 +4,7 @@
 #define pthread_create(A, B, C, D)
 #define pthread_join(A, B)
 #define pthread_cancel(A)
+#define pthread_testcancel()
 
 #define pthread_mutex_init(A, B)
 #define pthread_mutex_destroy(A)

lib/loop.c

@@ -256,6 +256,7 @@ int mosquitto_loop_forever(struct mosquitto *mosq, int timeout, int max_packets)
 
 	while(run){
 		do{
+			pthread_testcancel();
 			rc = mosquitto_loop(mosq, timeout, max_packets);
 		}while(run && rc == MOSQ_ERR_SUCCESS);
 		/* Quit after fatal errors. */
@@ -280,6 +281,7 @@ int mosquitto_loop_forever(struct mosquitto *mosq, int timeout, int max_packets)
 			return rc;
 		}
 		do{
+			pthread_testcancel();
 			rc = MOSQ_ERR_SUCCESS;
 			state = mosquitto__get_state(mosq);
 			if(state == mosq_cs_disconnecting || state == mosq_cs_disconnected){