pgsql: Enable building contrib/xml2 if configured using --with-libxml.

Log Message:
-----------
Enable building contrib/xml2 if configured using --with-libxml.
If this breaks things due to missing libxslt, then I'll have to
revert it, but let's see if it breaks the buildfarm.

Workarounds in case libxslt is missing include:
. don't configure with libxml, or
. don't build contrib modules from the contrib Makefile (use the individual module Makefiles instead), or
. change the xml2 Makefile

Modified Files:
--------------
    pgsql:
        configure (r1.542 -> r1.543)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/configure.diff?r1=1.542&r2=1.543)
        configure.in (r1.509 -> r1.510)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/configure.in.diff?r1=1.509&r2=1.510)
    pgsql/contrib:
        Makefile (r1.71 -> r1.72)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/contrib/Makefile.diff?r1=1.71&r2=1.72)
    pgsql/src:
        Makefile.global.in (r1.233 -> r1.234)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/Makefile.global.in.diff?r1=1.233&r2=1.234)