[ccache] Build Safety

[Dalton, Tom]

Thanks Vincent, that is very useful. I have one more clarification on
why I think using a shared cache will be useful (if it works!)
 
We have several sub-projects, A, B, C, D.
Our developers (and their development/build machines) can move around
between the sub-projects.
 
 
Subproject A and B 1 use gcc3, subproject C and D use gcc4.
Subproject A uses libraryX v1, subprojects B,C,D use libraryX v2.
 
So effectively there are 3 configurations, (gcc3 libX1), (gcc3 libX2),
(gcc4 libX2).
 
Now if each developer has a local cache, then each developer needs to do
each compile at least once, before the cache is populated. But if each
subproject has 10 developers, and the cache is shared, then only 1
developer needs to build the subproject, and then all the other
developers will get lots of cache hits when they compile.