library clean-up: Don't build libNX_Xpm anymore. Use system's libXpm shared library.

 One release goal for version 3.6.x of nx-libs is dropping as many bundled libraries as possible that haven't 
 been adapted to nx-libs.

 Starting with libNX_Xpm here.

 The libNX_Xpm library is only referenced once (nx-X11/programs/Xserver/hw/nxagent/Holder.c).
 When grepping through Xserver/hw/nxagent the suspicion comes up, that the libXpm linkage is not
 needed at all, because none of the provided functions are used inside the nxagent Xserver.