build: no bindings: make error more explicit

The panic that arises when there are no pre-build bindings available is
not that explicit and may confuse some users into thinking there's
something wrong with GDAL.

Signed-off-by: Frank Villaro-Dixon <frank@villaro-dixon.eu>

gdal-sys/build.rs

@@ -247,7 +247,7 @@ fn main() {
                 version.major, version.minor
             ));
             if !binding_path.exists() {
-                panic!("No pre-built bindings available for GDAL version {}.{}. Use `--features bindgen` to generate your own bindings.", version.major, version.minor);
+                panic!("No pre-built bindings available for GDAL version {}.{}. Use `--features bindgen` when adding this crate to generate your own bindings.", version.major, version.minor);
             }
 
             std::fs::copy(&binding_path, &out_path)