update gflags (#5904)

Author: jacquesqiao

cmake/external/gflags.cmake

@@ -28,15 +28,8 @@ INCLUDE_DIRECTORIES(${GFLAGS_INCLUDE_DIR})
 ExternalProject_Add(
     extern_gflags
     ${EXTERNAL_PROJECT_LOG_ARGS}
-    # TODO(yiwang): The annoying warnings mentioned in
-    # https://github.com/PaddlePaddle/Paddle/issues/3277 are caused by
-    # gflags.  I fired a PR https://github.com/gflags/gflags/pull/230
-    # to fix it.  Before it gets accepted by the gflags team, we use
-    # my personal fork, which contains above fix, temporarily.  Let's
-    # change this back to the official Github repo once my PR is
-    # merged.
-    GIT_REPOSITORY  "https://github.com/wangkuiyi/gflags.git"
-    GIT_TAG         986964c07427ecb9cdb5bd73f73ebbd40e54dadb
+    GIT_REPOSITORY  "https://github.com/gflags/gflags.git"
+    GIT_TAG         77592648e3f3be87d6c7123eb81cbad75f9aef5a
     PREFIX          ${GFLAGS_SOURCES_DIR}
     UPDATE_COMMAND  ""
     CMAKE_ARGS      -DCMAKE_CXX_COMPILER=${CMAKE_CXX_COMPILER}