Ccache depend mode

[Rene Engelhard]

On Tue, May 07, 2019 at 11:32:12AM +0200, Luboš Luňák wrote:
> - We use -MMD, which exludes system headers (or even our externals, since for 
> those we use -isystem too). This means that ccache could give incorrect hits 
> if those headers change. That may seem bad, but I think it's unlikely to 
> cause problems in practice, for several reasons:
>  * System headers rarely change.

They change all the time.

>  * If they change, it's generally a binary compatible change.

No? There's many versions of the same compatible header where the header
stays the same?

You seem to only think of people only developing on their stable, not
changing distro.

People often build stuff for their distros (as distro packagers, where
this stuff *does* change - and that also compatibly because said new update
affected someting else or a specific header of even the same header but
compatibly.

In development times, I upgrade my unstable chroot daily (and whatever changed
in unstable changes there, too. That includes system libs/headers and compilers
and whatever.)

Regards,

Rene