Add X11 XInput2 input plugin, including MPX and with support for axis controls.

Based on the original XLib plugin, and thus shares some of its warts.

CMakeTests/FindX11XInput2.cmake

@@ -0,0 +1,30 @@
+# For finding the X11 XInput extension, version 2. Because the standard CMake
+# FindX11.cmake may only know to look for version 1. Newer versions of CMake
+# than 2.8.7 might not have this problem, I wouldn't know. Better to be safe.
+
+
+if(USE_X11)
+    
+    IF(NOT X11_FOUND)
+        INCLUDE(FindX11)
+    ENDIF(NOT X11_FOUND)
+
+    IF(X11_FOUND)
+        FIND_PATH(X11_Xinput2_INCLUDE_PATH X11/extensions/XInput2.h ${X11_INC_SEARCH_PATH})
+        
+        FIND_LIBRARY(X11_Xinput2_LIB Xi ${X11_LIB_SEARCH_PATH})
+        
+        IF (X11_Xinput2_INCLUDE_PATH AND X11_Xinput2_LIB)
+            SET(X11_Xinput2_FOUND TRUE)
+            SET(X11_INCLUDE_DIR ${X11_INCLUDE_DIR} ${X11_Xinput2_INCLUDE_PATH})
+            message("X11 Xinput2 found")
+        ENDIF (X11_Xinput2_INCLUDE_PATH AND X11_Xinput2_LIB)
+        
+        MARK_AS_ADVANCED(
+            X11_Xinput2_INCLUDE_PATH
+            X11_Xinput2_LIB
+        )
+        
+    ENDIF(X11_FOUND)
+
+endif()