Art: don't update the header if not needed to avoid unnecessary recompiles

art/header.bash

@@ -0,0 +1,10 @@
+#!/bin/bash
+
+header=$(cat $1 |grep "=" |sed -e "s/ =.*/;/" -e "s/^/extern /")
+
+echo "$header" | diff - $2 > /dev/null 2>&1
+if [[ $? -ne 0 ]]
+  then
+  echo "Making header $2"
+  echo "$header" > $2
+fi

art/rules.mk

@@ -20,5 +20,4 @@ $(art_SPR_SRC): $(art_SPR_DATA)
 	@cat $^ | sed "s/^static uint16_t/unsigned short/" >> $@
 
 $(art_SPR_HDR): $(art_SPR_SRC)
-	@echo "Making header sprites.h"
-	@cat $< | grep "=" | sed -e "s/ =.*/;/" -e "s/^/extern /" > $@
+	@./$(art_LOCAL_PATH)/header.bash $< $@