test(x11): Warn for missing Xvfb program

Xvfb is required for *running* X11 tests. We do not make it mandatory
in the meson setup though, because we did not find a way to require a
*run* time dependency.

meson.build

@@ -769,6 +769,10 @@ test(
     env: test_env,
 )
 if get_option('enable-x11')
+    has_xvfb = find_program("Xvfb", required=false)
+    if not has_xvfb.found()
+        warning('Xvfb program not found, but is required to run X11 tests.')
+    endif
     test(
         'x11',
         executable('test-x11', 'test/x11.c', dependencies: x11_test_dep),

test/xvfb-wrapper.c

@@ -67,7 +67,6 @@ xvfb_wrapper(int (*test_func)(char* display))
         goto err_display_fd;
     }
     snprintf(display_fd_string, sizeof(display_fd_string), "%d", fileno(display_fd));
-    fprintf(stderr, "display_fd_string: %s\n", display_fd_string);
 
     /* Set SIGUSR1 to SIG_IGN so Xvfb will send us that signal
      * when it's ready to accept connections */
@@ -92,6 +91,11 @@ xvfb_wrapper(int (*test_func)(char* display))
     ret = posix_spawnp(&xvfb_pid, "Xvfb", NULL, NULL, xvfb_argv, envp);
     if (ret != 0) {
         fprintf(stderr, "posix_spawnp error %d: %s\n", ret, strerror(ret));
+        if (ret == ENOENT) {
+            fprintf(stderr,
+                    "Xvfb may be missing. Please install the corresponding "
+                    "package, e.g. \"xvfb\" or \"xorg-x11-server-Xvfb\".\n");
+        }
         ret = TEST_SETUP_FAILURE;
         goto err_xvfd;
     }