builtin is also an acid keyword.

Reference: /n/sources/patch/applied/ccacidkwd
Date: Sun Dec  5 14:23:27 CET 2010
Signed-off-by: quanstro@quanstro.net

--- /sys/src/cmd/cc/acid.c	Sun Dec  5 14:23:06 2010
+++ /sys/src/cmd/cc/acid.c	Sun Dec  5 14:23:05 2010
@@ -2,7 +2,7 @@
 
 static char *kwd[] =
 {
-	"$adt", "$aggr", "$append", "$complex", "$defn",
+	"$adt", "$aggr", "$append", "$builtin", "$complex", "$defn",
 	"$delete", "$do", "$else", "$eval", "$head", "$if",
 	"$local", "$loop", "$return", "$tail", "$then",
 	"$union", "$whatis", "$while",