Build rules

[kinook]

By design, build rules are evaluated for each step, even child steps of a build rule that was skipped (to accommodate the option for whether nesting of build rules and 'include in build' is turned on (Tools | Application Options | General). We're considering optimizing this for the case where nesting is enabled, but we do have to be careful about compatibility with previous versions. One way to get around this is to put the child step(s) with build rules in a subroutine, since the build rules in a subroutine won't get evaluated if the subroutine is not called.