tests: Provide memrchr() fallback for platforms that lack it

darwin does not provide memrchr(). Add a local fallback implementation
guarded by HAVE_MEMRCHR.

Signed-off-by: Jeremy Huddleston Sequoia <jeremyhu@apple.com>

meson.build

@@ -49,6 +49,7 @@ endforeach
 have_funcs = [
 	'accept4',
 	'getpeereid',
+	'memrchr',
 	'mkostemp',
 	'posix_fallocate',
 	'ppoll',

tests/queue-test.c

@@ -24,6 +24,7 @@
  */
 
 #define _GNU_SOURCE /* For memrchr */
+#include "../config.h"
 #include <stdlib.h>
 #include <stdint.h>
 #include <stdio.h>
@@ -36,6 +37,19 @@
 #include <assert.h>
 #include <signal.h>
 
+#ifndef HAVE_MEMRCHR
+static void *
+memrchr(const void *s, int c, size_t n)
+{
+	const unsigned char *p = (const unsigned char *)s + n;
+	while (n--) {
+		if (*--p == (unsigned char)c)
+			return (void *)p;
+	}
+	return NULL;
+}
+#endif
+
 #include "wayland-client.h"
 #include "wayland-server.h"
 #include "test-runner.h"