Decreasing link dependencies

Hi!

When building pgadmin3 on Linux, pgagent ends up with a whole lot of
depdencies on X and friends - since it pulls in all of wxWidgets. I want
to deploy pgagent to a server, but I certainly don't have X on my
servers... Building it statically removes the dependency on the wx
libraries, but the X libs stay in there.

Attach patch fixes this, by using a "base set" of libraries that doesn't
include wx (it does include libpq for exmaple, since anything in pgadmin
can be expected to require that). Then it adds the specific wx
requirements for pgadmin3 (loads of them) and pgagent (just base) using
*_LDADD. The same fix is done for libxml2 - pgagent doesn't need it (at
least not now).

//Magnus