Fix protobuf.cmake to handle empty PROTOBUF_VERSION correctly

Sometime FIND_PACKAGE(ProtoBuf) get an empty PROTOBUF_VERSION. In this case, ${PROTOBUF_VERSION} needs to be quoted.

cmake/external/protobuf.cmake

@@ -20,7 +20,7 @@ FIND_PACKAGE(Protobuf ${PROTOBUF_VERSION})
 IF(PROTOBUF_FOUND)
     EXEC_PROGRAM(${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")
+    IF ("${PROTOBUF_VERSION}" VERSION_LESS "3.1.0")
         SET(PROTOBUF_FOUND OFF)
     ENDIF()
 ENDIF(PROTOBUF_FOUND)