mapnik.go
func (m *Map) SetSRS(srs string) {
	cs := C.CString(srs)
	defer C.free(unsafe.Pointer(cs))
	C.mapnik_map_set_srs(m.m, cs)
}
func (m *Map) SetSRS(srs string) {
	cs := C.CString(srs)
	defer C.free(unsafe.Pointer(cs))
	C.mapnik_map_set_srs(m.m, cs)
}
// SetSRS sets the projection of the map as a proj4 string ('+init=epsg:4326', etc).