wayland: silence annoying libwayland-client message

libwayland-client contains the following code [1]:

	runtime_dir = getenv("XDG_RUNTIME_DIR");
	if (!runtime_dir) {
		fprintf(stderr,
			"error: XDG_RUNTIME_DIR not set in the environment.\n");

This means this message will unconditionally and unavoidably be printed
if XDG_RUNTIME_DIR is not set. Since mpv is a terminal program, and we
want to avoid unnecessary output, work it around by not attempting to
use wayland if this environment variable is not set.

[1] http://cgit.freedesktop.org/wayland/wayland/tree/src/wayland-client.c#n636
    (cd0dccd01e16fa404e03974d30ded3aebdb1c4bc)

0 files changed, 0 insertions, 0 deletions