Fix compiler warnings.

meta-cursor-renderer.c is already fixed upstream in a later release:

https://git.gnome.org/browse/mutter/commit?id=ff312b69585cb0ec9f53483baa47678db8d44306

Other two files need to be submitted upstream still.

--- mutter-3.18.0/src/backends/meta-cursor-renderer.c	Sun Oct  4 19:47:56 2015

+++ mutter-3.18.0/src/backends/meta-cursor-renderer.c	Sun Oct  4 19:48:42 2015

@@ -26,6 +26,7 @@

 #include "meta-cursor-renderer.h"

+#include <math.h> 

 #include <meta/meta-backend.h>

 #include <meta/util.h>

--- mutter-3.18.0/src/core/window.c	Mon Sep 21 12:48:51 2015

+++ mutter-3.18.0/src/core/window.c	Tue Jan  5 13:34:16 2016

@@ -3488,7 +3488,7 @@

     {

       MetaMonitorInfo *info = &window->screen->monitor_infos[i];

-      if (info->winsys_id == winsys_id)

+      if ((size_t)info->winsys_id == winsys_id)

         return info;

     }

@@ -3697,7 +3697,7 @@

   meta_window_update_monitor (window, flags & META_MOVE_RESIZE_USER_ACTION);

-  if (old_output_winsys_id != window->monitor->winsys_id &&

+  if (old_output_winsys_id != (size_t) window->monitor->winsys_id &&

       flags & META_MOVE_RESIZE_MOVE_ACTION && flags & META_MOVE_RESIZE_USER_ACTION)

     window->preferred_output_winsys_id = window->monitor->winsys_id;

--- mutter-3.18.1/src/x11/session.c	Tue Jan  5 13:40:50 2016

+++ mutter-3.18.1/src/x11/session.c	Tue Jan  5 13:41:09 2016

@@ -336,7 +336,7 @@

     prop3val.value = &hint;

     prop3val.length = 1;

-    sprintf (pid, "%d", getpid ());

+    sprintf (pid, "%ld", (long)getpid ());

     prop4.name = (char *)SmProcessID;

     prop4.type = (char *)SmARRAY8;

     prop4.num_vals = 1;