Improve incremental builds with unmodified GOB headers.

Improve incremental builds with unmodified GOB headers.

It's nice to not touch header files which are unchanged during an
incremental build, because that will trigger pointless rebuilds of
other files.  Pass --no-touch-headers to gob2 which enables this
behaviour.

Add a dist-hook to update the header timestamps when distributing
so that everything is fully up-to-date in the tarball.