Follow-up r297842: Rework header generation to fix always rebuilding.

This reworks the handling of common headers to just include the needed
logic rather than invoke MAKE. This avoids the problem listed in r297842
and avoids other dependency tracking issues.

Pointyhat to: bdrewery
Reported by: Nikolai Lifanov <lifanov@mail.lifanov.com>
Sponsored by: EMC / Isilon Storage Division