Right now fontconfig uses a cache found first in a path and
cachedirs are the order of the system-wide path and then the user path.
this is due to avoid writing caches into the user path when running as root.

However, changing caches by certain config only, e.g. using <match target="scan">
may not take effect by this behavior, because it may be stored into the user path.

Thus, needing to find the latest cache out from paths.

Fixes fontconfig/fontconfig#182