[ccache] direct mode design bug

[Mathias De Maré]

Hello all,

I believe ccache is an incredibly useful application (which is why I'm
introducing it at my work place).
Besides the great performance improvements for builds, I particularly
like this feature mentioned on the front page:

> Is it safe?
>
> Yes. The most important aspect of a compiler cache is to always produce exactly the same output that the real compiler would produce. This includes providing exactly the same object files and exactly the same compiler warnings that would be produced if you use the real compiler. The only way you should be able to tell that you are using ccache is the speed.

Ccache is really good at giving the same results as the actual
compiler. I know, there may be a few exceptions, but those should be
kept to a minimum.

Performance is an important aspect, but it seems better to me to
prefer the safe option (right now, that's preprocessing made, perhaps
in the future it can be a 'safe direct mode'), rather than the fast
option.
Application defaults (this is my personal opinion of course) benefit
from being safe. It's always possible for users to go with a more
advanced option if they do feel confident.

Greetings,
Mathias