Improving GDAL/OGR handling in extractorapp

- Check if native libs are available at bean instantiation
- Excluding some redundant jars at build time: having the gdal/ogr
  bindings twice could lead to version mismatches. In addition, we should use the
  generated bindings coming with the targeted native libs.
- Better logging if unavailable libs.

TODO:
- Propagate the modifications to other concerned webapps (mapfishapp,
  geoserver).

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