HPCC-9563 XML_CATALOG not being set correctly

Moved XML_CATALOG variable geneartion in xsltproc.cmake.in to config
generation time instead of at run time as it runs in a seperate cmake
thread during the build process.

Signed-off-by: Philip Schwartz <philip.schwartz@lexisnexis.com>
Merge pull request #4556 from pschwartz/HPCC-9563-xml-catalog-issue

HPCC-9563 XML_CATALOG not being set correctly

Reviewed-By: Richard Chapman <rchapman@hpccsystems.com>