[ccache] PATCH: Look at include files' mtimes

[Wilson Snyder]

>This patch lets ccache examine an include file's mtime and size in
>lieu of hashing it, during direct mode.  If the mtime and size don't
>match, we fall back to hashing.
>
>The net result is roughly a factor-of-two speedup in ccache hits (*),
>on my machine.
>
>I'm not sure if this is a desirable feature, because obviously mtimes
>can be tampered with.

IMO I at my site I would be reluctant to use ccache with
this enabled, and believe the default should be as safe as
possible. I've been burned by mtime only checking before as
(excluding some recent file systems) mtime has a resolution
only down to one second.

However including it as an option seems fine, and recording
it into the manifest also seems good.  I would also suggest
making the manifest format backward compatible when it is
easy as in this case; just test the version number when
reading the file.