Made XInput2 optional at compile-time.

This is required for RHEL 5, CentOS 5 and Cygwin-X.

Fixes #314.

src/x11_platform.h

@@ -43,15 +43,17 @@
 // The XRandR extension provides mode setting and gamma control
 #include <X11/extensions/Xrandr.h>
 
-// The XInput2 extension provides improved input events
-#include <X11/extensions/XInput2.h>
-
 // The Xkb extension provides improved keyboard support
 #include <X11/XKBlib.h>
 
 // The Xinerama extension provides legacy monitor indices
 #include <X11/extensions/Xinerama.h>
 
+#if defined(_GLFW_HAS_XINPUT)
+ // The XInput2 extension provides improved input events
+ #include <X11/extensions/XInput2.h>
+#endif
+
 #include "posix_tls.h"
 
 #if defined(_GLFW_GLX)
@@ -188,15 +190,6 @@ typedef struct _GLFWlibraryX11
         int         versionMinor;
     } xkb;
 
-    struct {
-        GLboolean   available;
-        int         majorOpcode;
-        int         eventBase;
-        int         errorBase;
-        int         versionMajor;
-        int         versionMinor;
-    } xi;
-
     struct {
         int         count;
         int         timeout;
@@ -215,6 +208,17 @@ typedef struct _GLFWlibraryX11
         int         versionMinor;
     } xinerama;
 
+#if defined(_GLFW_HAS_XINPUT)
+    struct {
+        GLboolean   available;
+        int         majorOpcode;
+        int         eventBase;
+        int         errorBase;
+        int         versionMajor;
+        int         versionMinor;
+    } xi;
+#endif /*_GLFW_HAS_XINPUT*/
+
 } _GLFWlibraryX11;