This fixes a build error occurring when building a configuration
having fontconfig enabled and `home.packages` only containing one
package installing things to `/lib`.
Also adds a number of test cases to verify the fontconfig cache
generation functionality.
Fixes#703
Also deprecates the `fonts.fontconfig.enableProfileFonts` option. The
configuration is now always generated if `fonts.fontconfig.enable` is
set.
Fixes#520
