This involves modifying the module to have a stable ABI, that is, the
xslt_process() function still exists even without libxslt. It throws a
runtime error if called, but doesn't prevent executing the CREATE FUNCTION
call. This is a good thing anyway to simplify cross-version upgrades.
I have changed the name of the new parse function to xml_valid and fixed
a reference to SortMem which meant that the code as supplied would work
against 7.3 and 7.4 but wouldn't work in CVS.