Build gdal with xercesc support for GML files

[mbarchfe@…]

If you call ogrinfo --formats it will list GML, but if you actually try to access a GML file, you will be surprised with the following error message:

ERROR 1: Unable to create Xerces C++ based GML reader, Xerces support not configured into GDAL/OGR.

With the following simple variant GML files could be read:

variant xerces {

depends_lib-append port:xercesc configure.args-append --with-xerces=${prefix}

}