touchui: First pass at XInput2 message pump.

build/linux/system.gyp

Index: build/linux/system.gyp
diff --git a/build/linux/system.gyp b/build/linux/system.gyp
index cf25bb11f797cbd9e13e2d264b48188b685b414c..7193c67792f80d1e73c1aeabc979f7fdc58c3151 100644
--- a/build/linux/system.gyp
+++ b/build/linux/system.gyp
@@ -225,7 +225,17 @@
               '<!@(<(pkg-config) --libs-only-l x11)',
             ],
           },
-      }]]
+      }],
+      # When XInput2 is available (i.e. inputproto version is 2.0), the
+      # pkg-config command will succeed, so the output will be empty.
+      ['"<!@(<(pkg-config) --atleast-version=2.0 inputproto || echo $?)"==""', {
+        'direct_dependent_settings': {
+          'defines': [
+            'HAVE_XINPUT2',
+          ],
+        },
+      }],
+      ],
     },
     {
       'target_name': 'xext',