config CHANGE timeout step is configurable

Fixes #164
diff --git a/src/config.h.in b/src/config.h.in
index 30dd8a3..a35d1bd 100644
--- a/src/config.h.in
+++ b/src/config.h.in
@@ -73,6 +73,11 @@
  */
 #define NC_PS_QUEUE_SIZE @MAX_PSPOLL_THREAD_COUNT@
 
+/* Microseconds after which tasks are repeated until the full timeout elapses.
+ * A millisecond (1000) should be divisible by this number without remain.
+ */
+#define NC_TIMEOUT_STEP @TIMEOUT_STEP@
+
 /* Portability feature-check macros. */
 #cmakedefine HAVE_PTHREAD_RWLOCKATTR_SETKIND_NP