Add HELP_RELAXNG & HELP_SCHEMA environment variables for XMLLINT

For instance, by path: ./configure HELP_RELAXNG=/usr/share/xml/docbook5/schema/rng/5.0/docbook.rng

For instance, by id: ./configure HELP_RELAXNG=http://docbook.org/xml/5.0/rng/docbook.rng

The id should be referenced in /etc/xml/catalog file. For instance:

/etc/xml/catalog file: ... ...

/usr/share/xml/docbook5/schema/rng/5.0/catalog.xml file: ... ...