Fix build error under go 1.3

When building under go 1.3, I get errors like this:

```
.local/src/github.com/go-gl/glu/glu.go:140: cannot use (*[0]byte)(q) (type
*[0]byte) as type *C.struct_GLUquadric in argument to _Cfunc_gluSphere
.local/src/github.com/go-gl/glu/glu.go:144: cannot use (*[0]byte)(q) (type
*[0]byte) as type *C.struct_GLUquadric in argument to _Cfunc_gluCylinder
.local/src/github.com/go-gl/glu/glu.go:148: cannot use (*[0]byte)(q) (type
*[0]byte) as type *C.struct_GLUquadric in argument to _Cfunc_gluDisk
.local/src/github.com/go-gl/glu/glu.go:152: cannot use (*[0]byte)(q) (type
*[0]byte) as type *C.struct_GLUquadric in argument to _Cfunc_gluPartialDisk
```

I'm slightly concerned this might break older versions of go. @shurcooL, any thoughts on that sort of thing?