Merge pull request #2680 from gangliao/protobu'f

FIX: Always build protobuf from source

cmake/external/protobuf.cmake

@@ -144,18 +144,7 @@ FUNCTION(build_protobuf TARGET_NAME BUILD_FOR_HOST)
 ENDFUNCTION()
 
 SET(PROTOBUF_VERSION 3.1)
-IF(NOT CMAKE_CROSSCOMPILING)
-    FIND_PACKAGE(Protobuf ${PROTOBUF_VERSION})
-
-    IF(PROTOBUF_FOUND)
-        SET_PROTOBUF_VERSION()
-        IF("${PROTOBUF_VERSION}" VERSION_LESS "3.1.0")
-            SET(PROTOBUF_FOUND OFF)
-        ELSE()
-            PROMPT_PROTOBUF_LIB()
-        ENDIF()
-    ENDIF(PROTOBUF_FOUND)
-ELSE()
+IF(CMAKE_CROSSCOMPILING)
     build_protobuf(protobuf_host TRUE)
     LIST(APPEND external_project_dependencies protobuf_host)