[ccache] base_dir and symbolic links

[Anders Björklund]

Andreas Wettstein wrote:
> In function `make_relative_path` (file `ccache.c`), the given
> path is made "canonic" before it is converted to a path relative
> to the current working directory.  In particular, "canonic" means
> that symbolic links are removed.  I understand that it makes
> sense to make the current working directory canonic, but I do not
> see why this done for the path given to `make_relative_path`.  Is
> it really necessary?
> 
> The reason why I am asking is that I have a usage scenario where
> this reduces sharing a cache among different users.

Seems like a pretty niche use case, but if you can come up with a
patch that doesn't break too much of current behaviour - why not.

You should still get preprocessor hits, but suppose you would
rather get "direct" hits instead of having to read both files.

Worst case, it could be an option (that defaulted to false) ?

Then you could override it at runtime, for your use case...

/Anders