[omniORB] Making the build system extensible

[Pfeiffer Daniel]

Hullo Duncan,

we use the omniORB build system for our own stuff.  This is very handy, as
it's got all the rules for IDL and so forth.  The drawback is that
everything gets exported into the bin, lib, ... directories of the omniORB
where other developers shouldn't be writing.

By changing beforedir from

EXPORT_TREE      = $(TOP)

to

EXPORT_TREE      ||= $(TOP)

we could easily override this in our build system.  Then every developer
could have his own little destination tree while using the same ORB.

Please Duncan, may we have this change?

thank you very much

Daniel