This commit exposes support for compilation without dbus, controlled
by the global dbusSupport argument. This argument is understood by
many other nixpkgs expressions and can be set globally in
~/.config/nixpkgs/config.nix.
This commit also adds two package-specific arguments:
* notificationsSupport allows to disable the desktop-notifications
feature when invoking cargo.
* useOpenSSL can be set to false in order to compile tiny with rustls
instead of openssl.
64cb3a021b