[ccache] basename of source file in hashed name in ccache

[Martin Pool]

I don't recall ccache ever having this on the main line.

If you think something has been corrupted (how?) you could probably
run something like 'rm -v $(grep -l foo ~/.ccache/?/?/*)', though
personally if something is corrupted I might blow away the whole
cache.

If you want to add this feature (probably controlled by an environment
option) just send a patch to this list.

Martin