Option to load X11 or Wayland libraries dynamically

[felselva]

With the compilation with support for both X11 and Wayland, it's required from the computer running SWFW to have both backends installed. This might be not the case for every Linux or FreeBSD machine out there. With that case in mind, it might be useful to support dynamic loading of the X11 and Wayland libraries when compiled to support both backends. However, this must be an option. The current setup must still exist.

This feature is not a priority.