* Skip font installation if the derivation hasn't changed.
* Use `rsync` instead of `install` to copy font files, to avoid useless
copying of pre-existent identical files.
Before, loading a module would be guarded by an optional platform
condition. This made it possible to avoid loading and evaluating a
module if it did not support the host platform.
Unfortunately, this made it impossible to share a single configuration
between GNU/Linux and Darwin hosts, which some wish to do.
This removes the conditional load and instead inserts host platform
assertions in the modules that are platform specific.
Fixes#1906
