xorg server: remove dependencies

Currently, the X.org server selection always trigger the compilation
and installation of libXt (the X toolkit) and libXaw (Athena Widget
toolkits), which is are legacy toolkits not used by modern toolkits
(Gtk, Qt, etc.). They are only used by old X11 applications that are
typically not used on embedded systems.

Not compiling them save compilation time, and more importantly, space
on the target.

We also remove the compilation on xlib_libxkbui, libxmu and libxpm,
because they all depend on libXt.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>