Correct Handle Python Proto2

cmake/external/python.cmake

@@ -26,10 +26,10 @@ IF(PYTHONLIBS_FOUND AND PYTHONINTERP_FOUND)
     find_python_module(wheel REQUIRED)
     find_python_module(google.protobuf REQUIRED)
     FIND_PACKAGE(NumPy REQUIRED)
-    IF(${PY_GOOGLE.PROTOBUF_VERSION} VERSION_LESS "3.0.0")
+    IF(${PY_GOOGLE.PROTOBUF_VERSION} AND ${PY_GOOGLE.PROTOBUF_VERSION} VERSION_LESS "3.0.0")
         MESSAGE(FATAL_ERROR "Found Python Protobuf ${PY_GOOGLE.PROTOBUF_VERSION} < 3.0.0, "
-        "please use pip to upgrade protobuf.")
-    ENDIF(${PY_GOOGLE.PROTOBUF_VERSION} VERSION_LESS "3.0.0")
+        "please use pip to upgrade protobuf. pip install -U protobuf")
+    ENDIF()
 ELSE(PYTHONLIBS_FOUND AND PYTHONINTERP_FOUND)
     MESSAGE(FATAL_ERROR "Please install python 2.7 before building PaddlePaddle.")
     ##################################### PYTHON ########################################