build system : support multiple generated folders

[mmadia]

As the number of possible generated targets has increased over the years, It'd be nice if the build system would have the option to organize the user's generated directories. eg,

generated:

hrev5 bone zeta haiku

gcc2 gcc4

This would be especially useful for those who build and keep multiple targets on the same system. This would also lay some foundation for creating software similar to Mozilla's Tinderboxes : ​http://www.mozilla.org/tinderbox.html , ​http://tinderbox.mozilla.org/

[bonefish]

The build system already supports multiple output directories. You can create an arbitrary directory and invoke the configure script from there. I usually create them in the root folder of the sources and call them generated-{gcc2,gcc4,ppc,test,...}, but you're free to create the hierarchy you proposed.

To build in a specific output directory, you have to cd there to invoke "jam", or alternatively set the HAIKU_OUTPUT_DIR environment variable an run "jam" from within the source tree.