Fix build error caused by CMAKE_CXX_FLAGS (#3204) #2440

Job	Run time
compile-with-cmake	16m 4s
compile-with-make	14m 29s
gcc-compile-with-bazel-all-options	11m 27s
gcc-compile-with-bazel	10m 51s
gcc-compile-with-boringssl	11m 25s
bazel-bvar-unittest	19m 1s
clang-compile-with-boringssl	16m 24s
clang-compile-with-bazel-all-options	16m 8s
clang-compile-with-bazel	15m 1s
clang-compile-with-make-protobuf	17m 1s
clang-unittest-asan	23m 42s
gcc-compile-with-make-protobuf	17m 12s
clang-unittest	17m 56s
	3h 26m 41s

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