Begin by setting up pbuilder (this generally needs to be done just once):
sudo pbuilder create --mirror http://us.archive.ubuntu.com/ubuntu --distribution dapper \
--othermirror "deb http://linuxcnc.org/emc2/ dapper emc2" \
--othermirror "deb http://us.archive.ubuntu.com/ubuntu/ dapper universe multiverse"
In the emc2.1 source directory, configure and make package the source:
(cd debian; ./configure sim) # or ./configure -r for realtime fakeroot dpkg-buildpackage -S -us
Now run pbuilder:
sudo pbuilder build emc2_2.1.0~alpha0.dsc
If any build-time dependencies were not specified in debian/control, the build will fail. On 2006/01/20, the 2.1 branch and "./configure sim" built successfully.