Let rgbasm handle undefined version constants.

blue.asm

@@ -1,2 +1 @@
 _BLUE EQU 1
-_RED  EQU 0

constants.asm

@@ -1,6 +1,4 @@
-_JAPAN EQU 0
-_GREEN EQU 0
-_YELLOW EQU 0
+INCLUDE "version.asm"
 
 INCLUDE "macros.asm"
 

red.asm

@@ -1,2 +1 @@
-_RED  EQU 1
-_BLUE EQU 0
+_RED EQU 1

version.asm

@@ -0,0 +1,20 @@
+if !def(_RED)
+_RED    EQU 0
+endc
+
+if !def(_BLUE)
+_BLUE   EQU 0
+endc
+
+if !def(_JAPAN)
+_JAPAN  EQU 0
+endc
+
+if !def(_GREEN)
+_GREEN  EQU 0
+endc
+
+if !def(_YELLOW)
+_YELLOW EQU 0
+endc
+