xwayland: check executable exists on init

Instead of walking PATH like a previous proposal [1], this one
checks that the Xwayland path specified in the pkg-config file
exists.

I think this is a reasonable compromise:

- Users that don't have Xwayland installed system-wide won't get
  a bogus DISPLAY env variable set up.
- Users that have WLR_XWAYLAND set won't be affected by this check.
- Users that have Xwayland installed system-wide and a different
  Xwayland in their PATH still get their custom Xwayland.
- Users that don't have Xwayland installed system-wide but have it
  somewhere else in PATH are left out. But this is pretty niche,
  and they can just set WLR_XWAYLAND.

[1]: https://github.com/swaywm/wlroots/pull/2314

xwayland/server.c

@@ -16,6 +16,7 @@
 #include <wlr/xwayland.h>
 #include "sockets.h"
 #include "util/signal.h"
+#include "xwayland/config.h"
 
 static void safe_close(int fd) {
 	if (fd >= 0) {
@@ -442,6 +443,11 @@ void wlr_xwayland_server_destroy(struct wlr_xwayland_server *server) {
 struct wlr_xwayland_server *wlr_xwayland_server_create(
 		struct wl_display *wl_display,
 		struct wlr_xwayland_server_options *options) {
+	if (!getenv("WLR_XWAYLAND") && access(XWAYLAND_PATH, X_OK) != 0) {
+		wlr_log(WLR_ERROR, "Cannot find Xwayland binary \"%s\"", XWAYLAND_PATH);
+		return NULL;
+	}
+
 	struct wlr_xwayland_server *server =
 		calloc(1, sizeof(struct wlr_xwayland_server));
 	if (!server) {