copying a timestamp

[Ralf Wildenhues]

* Joern Rennecke wrote on Sun, Nov 14, 2010 at 04:35:02PM CET:
> I just realized there is a third alternative: we could exploit the fact
> that the same file is handled by make as different targets if named
> with different pathnames, i.e.
> [$(srcdir)/doc/]tm.texi and $(srcdir)/doc/../doc/tm.texi are
> different make targets.
> I'm not sure if that would be exploiting a stable feature or a bug/limitation
> that might disappear from GNU make before the FSF GPL/GFDL problems
> are resolved.

It's not likely that this will disappear, but code like that will jump
out at the casual reader months from now as likely-buggy.  So that would
definitely deserve a comment.  Maybe also a make-time (or
configure-time) test that this feature/bug has not changed?

Note also that in case you're playing a similar game with ./file vs.
$(srcdir)/file that will hurt in-tree builds of GCC (which, while
advised against, would still be nice to have working).

Cheers,
Ralf