Recognize gccgo error message.

   explicit.go:21:5: error: incompatible types in assignment (need explicit conversion)
   explicit.go:26:5: error: incompatible types in assignment (type has no methods)
   explicit.go:27:5: error: incompatible types in assignment (need explicit conversion)
   explicit.go:30:6: error: incompatible types in assignment (need explicit conversion; missing method ‘N’)
   explicit.go:33:7: error: invalid type conversion (need explicit conversion; missing method ‘N’)
   explicit.go:36:5: error: incompatible types in assignment

R=rsc
DELTA=1  (0 added, 0 deleted, 1 changed)
OCL=31805
CL=31807

test/interface/explicit.go

@@ -37,5 +37,5 @@ func main() {
 	i2 = I2(i);	// ERROR "need explicit|need type assertion"
 
 	e = E(t);	// ok
-	t = T(e);	// ERROR "need explicit|need type assertion"
+	t = T(e);	// ERROR "need explicit|need type assertion|incompatible"
 }