[ccache] PATCH: Look at include files' mtimes

[Justin Lebar]

> Better to do 2 seconds, since FAT (and maybe some other Windows
> related setups) has only a 2-second resolution.
>
> The other thing you can do is, on Unix, use the latest of ctime and
> mtime, which should catch cases where the mtime gets reset.

Thanks for the tips!

I'm happy to update the patch, but I'd first want to hear Joel's thoughts.

-Justin