This patch allows deb packages to be generated more cleanly by Makepkgs
than it currently allows. In particular, it handles the issue of having
.git files in the generated tarball, and also puts the debs in build/deb
when they're built, which is a bit more convenient and consistent.
Another handy side-effect of doing the build this new way (pcp packages
are also built this way, FWIW), is that it verifies the SRCFILES macros
as part of the build, which ensures the source tarball that we generate
will actually build.