Recognize gccgo error message:

interface1.go:29:6: error: incompatible type in initialization (missing method Next)

R=rsc
DELTA=1  (0 added, 0 deleted, 1 changed)
OCL=18183
CL=18271

test/interface1.go

@@ -30,6 +30,6 @@ func AddInst(Inst) *Inst {
 func main() {
 	re := new(Regexp);
 	print("call addinst\n");
-	var x Inst = AddInst(new(Start));	// ERROR "illegal"
+	var x Inst = AddInst(new(Start));	// ERROR "illegal|incompatible"
 	print("return from  addinst\n");
 }