Change gflag.cmake to use default repo

[wangkuiyi]

Currently, our gflags.cmake file refers to my personal fork of gflags:

Paddle/cmake/external/gflags.cmake

Lines 31 to 39 in 1c31bb9

	# 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

This is because of a bug in the official gflags repo as mentioned in #3277. I filed a PR gflags/gflags#230 to fix the bug, and it seems that gflags maintainers have merged it. So it is the time for us to switch back using the official gflags repo.

[Xreki]

Now the official gflags repo is used in Paddle, see

Paddle/cmake/external/gflags.cmake

Lines 28 to 47 in 5c500c6

	ExternalProject_Add(
	extern_gflags
	${EXTERNAL_PROJECT_LOG_ARGS}
	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}
	-DCMAKE_C_COMPILER=${CMAKE_C_COMPILER}
	-DCMAKE_CXX_FLAGS=${CMAKE_CXX_FLAGS}
	-DCMAKE_C_FLAGS=${CMAKE_C_FLAGS}
	-DCMAKE_INSTALL_PREFIX=${GFLAGS_INSTALL_DIR}
	-DCMAKE_POSITION_INDEPENDENT_CODE=ON
	-DBUILD_TESTING=OFF
	-DCMAKE_BUILD_TYPE=${THIRD_PARTY_BUILD_TYPE}
	${EXTERNAL_OPTIONAL_ARGS}
	CMAKE_CACHE_ARGS -DCMAKE_INSTALL_PREFIX:PATH=${GFLAGS_INSTALL_DIR}
	-DCMAKE_POSITION_INDEPENDENT_CODE:BOOL=ON
	-DCMAKE_BUILD_TYPE:STRING=${THIRD_PARTY_BUILD_TYPE}
	)