Log Message:
gtk3: Add Wayland support to options.mk.

This patch also allows multiple GDK backends to be selected (e.g. for
compiling with both X11 and Wayland support).

Previously only one backend could be enabled at a time, but the only
configuration where using multiple would be possible would be Quartz+X11
on macOS. I haven't tested this configuration on a macOS system, but it
would be interesting to know whether it works.

Builds with Wayland support on NetBSD...