Correct the definition of PROTOBUF_VERSION.

cmake/external/protobuf.cmake

@@ -75,6 +75,7 @@ FUNCTION(build_protobuf TARGET_NAME BUILD_FOR_HOST)
     LIST(APPEND external_project_dependencies ${TARGET_NAME} PARENT_SCOPE)
 ENDFUNCTION()
 
+SET(PROTOBUF_VERSION 3.1)
 IF(NOT CMAKE_CROSSCOMPILING)
     FIND_PACKAGE(Protobuf ${PROTOBUF_VERSION})
 
@@ -91,7 +92,6 @@ ELSE()
         CACHE FILEPATH "protobuf executable." FORCE)
 ENDIF()
 
-SET(PROTOBUF_VERSION 3.1)
 IF(NOT PROTOBUF_FOUND)
     build_protobuf(protobuf FALSE)
     SET(PROTOBUF_INCLUDE_DIR ${protobuf_INCLUDE_DIR}