Warn (and recover) from config file without <cachedir> elements.

Warn (and recover) from config file without <cachedir> elements.

When updating from older fontconfig versions, if the config file
is not replaced, it will not contain <cachedir> elements. Lacking these,
fontconfig has no place to store cached font information and cannot operate
reasonably.

Add code to check and see if the loaded configuration has no cache
directories, and if so, warn the user and add both the default system cache
directory and the normal per-user cache directory.