Prepare Ramps for user redefinition

Marlin/pins_RAMPS_13.h

 #define X_DIR_PIN          55
 #define X_ENABLE_PIN       38
 #define X_MIN_PIN           3
-#define X_MAX_PIN           2
+#ifndef X_MAX_PIN
+  #define X_MAX_PIN         2
+#endif
 
 #define Y_STEP_PIN         60
 #define Y_DIR_PIN          61
 
 #if ENABLED(REPRAP_DISCOUNT_SMART_CONTROLLER) || ENABLED(G3D_PANEL)
   #define KILL_PIN         41
-#else
-  #define KILL_PIN         -1
 #endif
 
 #if MB(RAMPS_13_EFF)
   #define MISO_PIN         50
   #define MOSI_PIN         51
 #endif
+
+#ifndef KILL_PIN
+  //  #define KILL_PIN         -1
+#endif