[5] author では高々1つ、
それ以外では任意個、使うことができます >>1。
[6] GPX - GPS Exchange Format ( 版) <http://www.gdal.org/drv_gpx.html>
By default, up to 2 <link> elements can be taken into account by feature. This default number can be changed with the GPX_N_MAX_LINKS environment variable.