build: wire-up wayland-egl

Wire-up the imported sources, test and pkg-config files.

v2:
 - Don't mangle with existing EXTRA_DIST list
 - Add the symbols check script to the `make check' target
 - Rename wayland-egl-{priv,backend}.h

Signed-off-by: Emil Velikov <emil.velikov@collabora.com>
Reviewed-by: Arnaud Vrac <avrac@freebox.fr>