workaround brew changing docbook uris in the catalog (#1536)

by using our local copy of the schema.

Author: tsteven4

tools/make_gpsbabel_doc.sh

@@ -2,7 +2,7 @@
 set -ex
 
 perl xmldoc/makedoc
-xmllint --noout --relaxng http://docbook.org/xml/5.0/rng/docbook.rng xmldoc/readme.xml
+xmllint --noout --relaxng "file:///$(pwd)/tools/schema/docbook/xml/5.0/rng/docbook.rng" xmldoc/readme.xml
 # the following doesn't seem to work.
 #xmllint --noout --schematron http://docbook.org/xml/5.0/sch/docbook.sch xmldoc/readme.xml
 # jing and many depedencies removed from fedora