gccgo: does not error on unused var

gccgo successfully compiles the following incorrect program:

``` go
package a
func g() {
    var f = func() {}
}
```

Both gc and gotype say:

```
test.go:3:6: f declared but not used
```

gofrontend is on commit 81810917af7ba19e1f9f8efc8b1989f7d6419d30
