build: make default backend configurable

Instead of hardcoding drm-backend.so as the default if environment
presents neither Wayland nor X11, have a ./configure option to change
it. It still defaults to drm-backend.so, if not given.

Signed-off-by: Pekka Paalanen <ppaalanen@gmail.com>

src/compositor.c

@@ -3151,7 +3151,7 @@ int main(int argc, char *argv[])
 		else if (getenv("DISPLAY"))
 			backend = "x11-backend.so";
 		else
-			backend = "drm-backend.so";
+			backend = WESTON_NATIVE_BACKEND;
 	}
 
 	config_file = config_file_path("weston.ini");