pgsql: Fix contrib/xml2 so regression test still works when it's built

Log Message:
-----------
Fix contrib/xml2 so regression test still works when it's built without libxslt.

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.

Tags:
----
REL8_2_STABLE

Modified Files:
--------------
pgsql/contrib/xml2:
Makefile (r1.8.2.2 -> r1.8.2.3)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/contrib/xml2/Makefile?r1=1.8.2.2&r2=1.8.2.3)
pgxml.sql.in (r1.9 -> r1.9.2.1)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/contrib/xml2/pgxml.sql.in?r1=1.9&r2=1.9.2.1)
xslt_proc.c (r1.7.2.3 -> r1.7.2.4)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/contrib/xml2/xslt_proc.c?r1=1.7.2.3&r2=1.7.2.4)

Added Files:
-----------
pgsql/contrib/xml2/expected:
xml2_1.out (r1.1.6.1)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/contrib/xml2/expected/xml2_1.out?rev=1.1.6.1&content-type=text/x-cvsweb-markup)