when loading a geotiff must use XTIFFClientOpen

when loading a geotiff we must use XTIFF* functions supplied by geotiff/xtiff.h. changed vil_tiff.cxx:open_tiff() to use XTIFFClientOpen if we HAVE_GEOTIFF. XTIFFClientOpen simply declares some new headers before calling through to TIFFClientOpen, so loading a tiff image with it is fine.
modified the configure_file slightly
check the status code of get_key_value() and return accordingly