I thought I did. The 'build' target should do everything that doesn't require root (which the install and binary targets do, because files should be owned by root).
The problem is rather complicated. It lies in the fact that the autobuilders always invoke the build target of debian/rules, which does all building of arch-specific as well as arch-independent stuff. Optional build-arch and build-indep targets are defined in policy, but the fact that they are optional and that there is no robust way to check for their existance means that can't be used in the general case. Technically nothing has to be done by the build target though, since it's the binary-arch and binary-indep targets that complete the package building.