[Build-common-hackers] Bug#284428: cdbs: allow use of kde.mk without DEB_BUILDDIR set

[Daniel Glassey]

> kde.mk automatically sets DEB_BUILDDIR = obj-$(DEB_BUILD_GNU_TYPE).
> But some packages do not build cleanly with a separate build directory.
> Unfortunately, it is apparently not possible to unset DEB_BUILDDIR
> before or after including kde.mk.  Therefore, I have to resolve to
> locally hacked copies of kde.mk.  It would be nice if using kde.mk
> without DEB_BUILDDIR set were possible.

This also meant that you can't use tarball.mk with kde.mk since
tarball.mk sets DEB_BUILDDIR.

Regards,
Daniel