installation domains

[Richard Frith-Macdonald]

Tentative policy and development proposals ... please enhance and then  
we can see if we can agree on the way forward.

1. We will move to FHS as the default layout for new installations

2. We will retain backward compatibility for existing installations  
(at least for a year or two)

To maintain backward compatibility, the current 'system' applications  
will need to remain as such when installed into an existing system.

That means they need to carry on to have GNUSTEP_INSTALLATION_DOMAIN  
defined as SYSTEM in their makefiles (or be changed to use another  
name, but if we do that we need to synchronise the change with a  
gnustep-make change, so it's probably easier to leave them unchanged).

3. gnustep-make needs to be changed to:
a. continue to honor GNUSTEP_INSTALLATION_DOMAIN in the makefiles if  
it is installing on an existing system using old-style locations.
b. ignore GNUSTEP_INSTALLATION_DOMAIN in makefiles if it is installing  
into a new system or a system already using a non-gnustep style layout.

4. gnustep-make should probably also warn about the consequences of  
the layout selected

5. documentation should be updated to use FHS style layout in all  
examples.

Does that cover it?