[haiku-appserver] Jamfile changes

Hi,

I've slightly changed the way the app_server is built.
The RUN_WITHOUT_REGISTRAR stuff is still missing though, but a 
"TARGET_PLATFORM=haiku jam app_server" does work now (from anywhere).

Before the change, I could not build the app_server for R5, and it 
still fails, although the objects are now built correctly (which was 
not the case due to the drawing/ subdirectory in the path name).

I also changed the DISPLAYDRIVER option to be a true compile time 
option - that way, I could remove parts of the app_server that are 
(hopefully) not essential for a given display driver. I've not tested 
this, though.

If you have any more suggestions or fixes, feel free to share.

Bye,
   Axel.

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