clean up, add expansion options

Author: thinkyhead

Marlin/src/inc/SanityCheck.h

@@ -626,10 +626,12 @@ static_assert(COUNT(arm) == LOGICAL_AXES, "AXIS_RELATIVE_MODES must contain " _L
  */
 #if HAS_MULTI_EXTRUDER
 
-  #if HAS_EXTENDABLE_MMU
-    #define MAX_EXTRUDERS 15
-  #else
-    #define MAX_EXTRUDERS  8
+  #ifndef MAX_EXTRUDERS
+    #if HAS_EXTENDABLE_MMU
+      #define MAX_EXTRUDERS 15
+    #else
+      #define MAX_EXTRUDERS  8
+    #endif
   #endif
   static_assert(EXTRUDERS <= MAX_EXTRUDERS, "Marlin supports a maximum of " STRINGIFY(MAX_EXTRUDERS) " EXTRUDERS.");
   #undef MAX_EXTRUDERS