Share X11 fd polling logic with Wayland

This moves the X11 polling implementation to a separate file where it
can be used by either the X11 or Wayland backend or both.

This code should be POSIX compatible where necessary but will use the
lower latency but non-standard polling functions ppoll or pollts where
those are available.

This commit is based on work by OlivierSohn and kovidgoyal.

Fixes #1281
Closes #1285

src/x11_platform.h

@@ -453,6 +453,7 @@ typedef VkResult (APIENTRY *PFN_vkCreateXcbSurfaceKHR)(VkInstance,const VkXcbSur
 typedef VkBool32 (APIENTRY *PFN_vkGetPhysicalDeviceXcbPresentationSupportKHR)(VkPhysicalDevice,uint32_t,xcb_connection_t*,xcb_visualid_t);
 
 #include "xkb_unicode.h"
+#include "posix_poll.h"
 
 #define GLFW_X11_WINDOW_STATE           _GLFWwindowX11 x11;
 #define GLFW_X11_LIBRARY_WINDOW_STATE   _GLFWlibraryX11 x11;