contrib/xml2 can get by without libxslt; the relevant features just
won't work. But if doesn't have libxml2, or if sepgsql doesn't have
libselinux, the link succeeds but the module then fails to work at load
time. To avoid that, link the require libraries unconditionally, so
that it will be clear at link-time that there is a problem.
Per discussion with Tom Lane and KaiGai Kohei.
This is still pretty rough - among other things, the documentation
needs work, and the messages need a visit from the style police -
but this gets the basic framework in place.
KaiGai Kohei
