ENH: add warning for openCV versions

Modules/ThirdParty/OpenCV/otb-module-init.cmake

 find_package ( OpenCV REQUIRED )
 mark_as_advanced( OpenCV_DIR )
+
+if("${OpenCV_VERSION}" VERSION_LESS "2.4.0")
+  message(WARNING "The OpenCV version detected (${OpenCV_VERSION}) is older than 2.4.0, "
+    "it may not be fully compatible with OTB")
+endif()