We are patching the xmlcatalog calls in the JH_CHECK_XML_CATALOG
macro to use the XML_CATALOG_FILES environment variable instead
of a single central catalog file. Since consumers (libraries or
applications) are expected to copy the m4 files to their source
tree, we need to patch them separately. The files should be
identical, though, so we can reuse the patch for gtk_doc.
This commit makes the patch available in passthru so the consumer
expressions do not need to maintain the long relative path.
a6cd26740b