time: Change TIME_HAVE_MONOTONIC to be 0/1 rather than defined/undefined

Generally, ccan config variables are always defined as either 0 or 1 and
are tested with #if.  That's instead of being being either defined or
undefined and tested with #ifdef.

TIME_HAVE_MONOTONIC breaks that convention.  This can cause warnings in
ccan/timer which uses it assuming the 0/1 convention.  Change it to remove
that warning.
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>

ccan/time/time.c

@@ -28,7 +28,7 @@ struct timeabs time_now(void)
 struct timemono time_mono(void)
 {
 	struct timemono ret;
-#ifdef TIME_HAVE_MONOTONIC
+#if TIME_HAVE_MONOTONIC
 	clock_gettime(CLOCK_MONOTONIC, &ret.ts);
 #else /* Best we can do */
 	ret.ts = time_now().ts;

ccan/time/time.h

@@ -71,6 +71,8 @@ struct timemono {
  */
 #if HAVE_CLOCK_GETTIME && defined(CLOCK_MONOTONIC)
 #define TIME_HAVE_MONOTONIC 1
+#else
+#define TIME_HAVE_MONOTONIC 0
 #endif
 
 struct timespec time_check_(struct timespec in, const char *abortstr);