Moved custom_synthetic.dmi into a proper define.

code/modules/mob/living/silicon/ai/ai.dm

@@ -278,7 +278,7 @@ var/list/ai_verbs_default = list(
 
 			if(Entry[1] == src.ckey && Entry[2] == src.real_name)
 				custom_sprite = 1 //They're in the list? Custom sprite time
-				icon = 'icons/mob/custom-synthetic.dmi'
+				icon = CUSTOM_ITEM_ROBOT
 
 		//if(icon_state == initial(icon_state))
 	var/icontype = ""

code/modules/mob/living/silicon/robot/custom_sprites.dm

@@ -25,6 +25,6 @@ var/list/robot_custom_icons
 	var/rname = robot_custom_icons[ckey]
 	if(rname && rname == real_name)
 		custom_sprite = 1
-		icon = 'icons/mob/custom-synthetic.dmi'
+		icon = CUSTOM_ITEM_ROBOT
 		if(icon_state == "robot")
 			icon_state = "[ckey]-Standard"

code/setup.dm

@@ -977,6 +977,9 @@ var/list/be_special_flags = list(
 #ifndef CUSTOM_ITEM_MOB
 #define CUSTOM_ITEM_MOB 'icons/mob/custom_items_mob.dmi'
 #endif
+#ifndef CUSTOM_ITEM_ROBOT
+#define CUSTOM_ITEM_ROBOT 'icons/mob/custom_synthetic.dmi'
+#endif
 
 //default item on-mob icons
 #define INV_HEAD_DEF_ICON 'icons/mob/head.dmi'