Bug#827125: Build a split package for X11 and Wayland

[Emmanuel Gil Peyrot]

Package: libglfw3
Version: 3.2-1

The current version of GLFW supports both X11 and Wayland, but this is
a build-time choice.  It would be very useful for people on Wayland
systems to be able to install something like `libglfw3-wayland' which
would provide `libglfw3', and rename the current package to
`libglfw3-x11'.

The dependencies of the Wayland backend are libwayland-client0,
libwayland-cursor0, libwayland-egl1 and libxkbcommon0, while the build
dependencies are extra-cmake-modules, wayland-protocols.

This would be for both Stretch and Sid.