Make GUIDE_SPIRAL_TIME_LIMIT configurable

[Khalid Baheyeldin]

This is a small change to allow the user to override spiral search time limit in Config.h

diff --git a/Validate.h b/Validate.h
index 1d4cbee..ca4df9f 100644
--- a/Validate.h
+++ b/Validate.h
@@ -72,7 +72,9 @@
 #define SEPARATE_PULSE_GUIDE_RATE ON
 
 // default time for spiral guides is 103.4 seconds
-#define GUIDE_SPIRAL_TIME_LIMIT 103.4
+#ifndef GUIDE_SPIRAL_TIME_LIMIT
+  #define GUIDE_SPIRAL_TIME_LIMIT 103.4
+#endif
 
 // automatically set MaxRate from SLEW_RATE_BASE_DESIRED
 #define MaxRate ((1000000.0/SLEW_RATE_BASE_DESIRED)/AXIS1_STEPS_PER_DEGREE)