define this value if it's not in the system headers

--HG--
extra : convert_revision : svn%3Ac70aab31-4412-0410-b14c-859654838e24/trunk%403464

src/video/x11/SDL_x11mouse.c

@@ -122,6 +122,9 @@ X11_InitMouse(_THIS)
                     } else {
                         SDL_AddMouse(&mouse, DevList[i].name, 0, 0, 1);
                     }
+#ifndef IsXExtensionPointer
+#define IsXExtensionPointer 4
+#endif
                     if (DevList[i].use == IsXExtensionPointer) {
                         ++num_mice;
                     }