make the maximum number of retries configurable at compile time

using a preprocessor define like this: `CFLAGS="-Os -DNATPMP_MAX_RETRIES=4" make`.
The rationale is that users might want to reduce the maximum timeout of
127.75 seconds with an invalid gateway.

natpmp.c

@@ -286,7 +286,7 @@ NATPMP_LIBSPEC int readnatpmpresponseorretry(natpmp_t * p, natpmpresp_t * respon
 			gettimeofday(&now, NULL);	// check errors !
 			if(timercmp(&now, &p->retry_time, >=)) {
 				int delay, r;
-				if(p->try_number >= 9) {
+				if(p->try_number >= NATPMP_MAX_RETRIES) {
 					return NATPMP_ERR_NOGATEWAYSUPPORT;
 				}
 				/*printf("retry! %d\n", p->try_number);*/

natpmp.h

@@ -51,6 +51,13 @@ typedef unsigned short uint16_t;
 #endif	/* _WIN32 */
 #include "natpmp_declspec.h"
 
+/* Set to 9 by https://tools.ietf.org/html/rfc6886#section-3.1 which leads to a
+ * maximum timeout of 127.75 seconds, due to the initial 250 ms timeout doubling
+ * each time, so we allow a compile-time modification here.*/
+#ifndef NATPMP_MAX_RETRIES
+#define NATPMP_MAX_RETRIES (9)
+#endif
+
 typedef struct {
 	int s;	/* socket */
 	in_addr_t gateway;	/* default gateway (IPv4) */