[Fontconfig] Startup cache check

[u-pnrz at aetey.se]

On Tue, Jan 12, 2016 at 03:05:04PM +0300, LRN wrote:
> Is there a way to manually run a cache check at startup without rebuilding
> the cache?
> 
> I'd like to do that, and terminate the program if the cache is not up to
> date. The only alternative is to let the application run and let FC
> eventually rebuilt the cache, which can take minutes, depending on the
> number of fonts and the disk performance. I'd rather tell the user to

Because you for a good reason refer to disk performance:

I would also suggest to assume that the data is accessed over network and
make no assumptions about the actual speed of the access, which naturally
varies a lot and easily makes cache generation very expensive. (This makes
the cache even more valuable, to avoid those costs during the usage.)

Rune