Minimal port of xfbuild to D2

[Andrej Mitrovic]

Yeah my port is very much still alpha. I don't yet know what to do
about the fact that parallel builds are awfully broken. xfBuild uses
associative arrays and modifies them on-the-fly in multiple threads,
and this causes synchronization issues.

To be fair, at the time I didn't really know that Tango was going to
be ported to D2 and that it will work well, so in retrospect maybe
your port should be worked on instead of mine. I'm awfully busy with
other stuff right now anyways (like wxD). I didn't waste too much time
porting xfBuild (only took a few hours), so I'm totally ok if your
port is used instead of mine.

Anyway, nice job! :)