Fix getrandom() syscall not existing while function exists in header

Found by: jack3
Patch by: Cizzle

On systems running Linux older than 3.17 and using glibc 2.25 or newer, the getrandom() function exists in the system header but the system call does not exist. This checks and skips the call if not found.
(cherry picked from commit 2962dd2)

src/main.c

@@ -1021,12 +1021,20 @@ int mainloop(int toplevel)
 static void init_random(void) {
   unsigned int seed;
 #ifdef HAVE_GETRANDOM
-  if (getrandom(&seed, sizeof(seed), 0) != sizeof(seed))
-    fatal("ERROR: getrandom()\n", 0);
-#else
-  struct timeval tp;
-  gettimeofday(&tp, NULL);
-  seed = (tp.tv_sec * tp.tv_usec) ^ getpid();
+  if (getrandom(&seed, sizeof(seed), 0) != sizeof(seed)) {
+    if (errno != ENOSYS) {
+      fatal("ERROR: getrandom()\n", 0);
+    } else {
+      /* getrandom() is available in header but syscall is not!
+       * This can happen with glibc>=2.25 and linux<3.17
+       */
+#endif
+      struct timeval tp;
+      gettimeofday(&tp, NULL);
+      seed = (tp.tv_sec * tp.tv_usec) ^ getpid();
+#ifdef HAVE_GETRANDOM
+    }
+  }
 #endif
   srandom(seed);
 }