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.

Modified Files:
--------------
    pgsql/contrib/xml2:
        Makefile (r1.13 -> r1.14)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/contrib/xml2/Makefile?r1=1.13&r2=1.14)
        pgxml.sql.in (r1.11 -> r1.12)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/contrib/xml2/pgxml.sql.in?r1=1.11&r2=1.12)
        xslt_proc.c (r1.18 -> r1.19)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/contrib/xml2/xslt_proc.c?r1=1.18&r2=1.19)

Added Files:
-----------
    pgsql/contrib/xml2/expected:
        xml2_1.out (r1.1)

(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/contrib/xml2/expected/xml2_1.out?rev=1.1&content-type=text/x-cvsweb-markup)