[Fontconfig] performance issue questions

[Masamichi Hosoda]

> If your program detects duplicates updates the mtime of the directories,
> that triggers to rebuild caches as I said earlier. that isn't "nothing
> changed".
> 
> Anyway, we may need to think about this again. for instance, adding an API
> to enable/disable the cache updates automatically in fontconfig or get rid
> of it completely and giving a responsibility to the applications to do it
> may be good perhaps.

My patch makes that applications can find out
whether creating/updating caches are necessary or not.

https://lists.freedesktop.org/archives/fontconfig/2016-November/005854.html

I believe it can partially solve this problem.