[ccache] Caching failed compilations

[Akim Demaille]

Hi all,

I'm heavily using ccache in a project where we generate C++
and compile it.  The generated code itself is heavy too, and
it is not uncommon to get compilation errors after half a
minute of compilation.

Because we sometimes repeatedly fall on the same error, particularly
when we try to fix bugs, we have to wait several times for that
amount of times.

Could ccache offer a mode where even failed compiled be ccached?

Thanks!