copying a timestamp (Was: Ping^2: RFA (build): Avoid circular dependency)

[Joern Rennecke <amylaar at spamcop dot net>]

Quoting Joern Rennecke <amylaar@spamcop.net>:

> P.S.: Of course it's the other way round:
> doc/tm.texi depends on the stampfile,
Oops, there is the circular dependency again.

What we actually need is a timestamp that says that the consistency has been
checked, and which depends on doc/tm.texi - that's our current s-tm-texi -
plus a generated file that depends on s-tm-texi, but bears the file system
time stamp of doc/tm.texi .
I.e. the s-tm-texi rule should make this file.
Can we use cp -p for this portably?
In principle we could also use some combination of time and ls and shell
substitutions, that would use less disk space, but I fear the portability
issues there could be worse.

Or should I better try to use double-colon rules?

I.e. TEXI_GCCINT_FILES would include [doc/]tm.texi, but all the files that
depend on TEXI_GCCINT_FILES would do so with a double-colon rule, and in
an earlier double-colon rule, they'd all depend on s-tm-texi, with empty
commands, to make sure doc/tm.texi is up-to-date.
Using double-colon rules would avoid portability concerns how to copy a
timestamp, but would force as to write all the commands to generate the
files that depend on tm.texi in the rule that state the dependency.