gbuild support for external projects

Hi all,

I have created a new gbuild class UnpackedTarball that handles preparing
sources for external project: unpacking, patching and a bit more
(required by clucene). It is based on the custom target from module
clucene, which is itself based on Norbert's ExternalLib gbuild class, I
think (I have not reused ExternalLib, because I only wanted to handle
the preparation part). I also updated clucene and ucpp (the only two
external modules converted to gbuild so far) to use it. With this in
place, I hope we can relatively easily convert the dozen or so external
modules that are built by dmake (BUILD_ACTION=dmake) now.

As for the rest of the external modules, my current idea is to use
free-form makefile, like CustomTarget, for everything but preparation of
sources. That would match the current dmake makefiles pretty well, I
think...

The code lives on branch feature/gbuild_external. I am pretty confident
it will not break on Windows, but I will not mind testers :-) If there
are no violent objectives, I will merge it to master in 1-2 days.

D.