bug#18640: [wishlist] fontconfig's cache is not refreshed automatically

[Mark H Weaver]

When a user runs emacs (or any other program that uses fontconfig), a
cache is created in ~/.cache/fontconfig.  This cache is not updated
automatically after installing new fonts, and must be manually cleared
out.

I guess this might be related to our use of Jan 1, 1970 as the
timestamps for all files in /gnu/store (which includes user profiles).

We should either modify fontconfig to detect a stale cache using a
method that works on Guix, or else notify the user that they must clear
the cache after installing ttf fonts.

      Mark