CDRIVER-4234 Use arc4random_buf if available

Make arc4random_buf the first choice to use if it is available.
Previously, if rand_r was not available, the older rand would be used
even if arc4random_buf was available.

Author: ryandesign

src/libbson/src/bson/bson-context.c

@@ -225,14 +225,14 @@ static int32_t
 _get_rand (unsigned int *pseed)
 {
    int32_t result = 0;
-#ifndef BSON_HAVE_RAND_R
+#ifdef BSON_HAVE_ARC4RANDOM_BUF
+   arc4random_buf (&result, sizeof (result));
+#elif defined(BSON_HAVE_RAND_R)
+   result = rand_r (pseed);
+#else
    /* ms's runtime is multithreaded by default, so no rand_r */
    /* no rand_r on android either */
    result = rand ();
-#elif defined(BSON_HAVE_ARC4RANDOM_BUF)
-   arc4random_buf (&result, sizeof (result));
-#else
-   result = rand_r (pseed);
 #endif
    return result;
 }