Fix cmake find protobuf problem

Author: gangliao

cmake/external/protobuf.cmake

@@ -16,6 +16,14 @@ INCLUDE(ExternalProject)
 
 FIND_PACKAGE(Protobuf 3.1)
 
+IF(PROTOBUF_FOUND)
+    EXEC_PROGRAM(${PROTOBUF_PROTOC_EXECUTABLE} ARGS --version OUTPUT_VARIABLE PROTOBUF_VERSION)
+    STRING(REGEX MATCH "[0-9]+.[0-9]+" PROTOBUF_VERSION "${PROTOBUF_VERSION}")
+    IF (${PROTOBUF_VERSION} VERSION_LESS "3.1.0")
+        SET(PROTOBUF_FOUND OFF)
+    ENDIF()
+ENDIF(PROTOBUF_FOUND)
+
 IF(NOT PROTOBUF_FOUND)
     SET(PROTOBUF_SOURCES_DIR ${THIRD_PARTY_PATH}/protobuf)
     SET(PROTOBUF_INSTALL_DIR ${THIRD_PARTY_PATH}/install/protobuf)