option(YAML_CPP_BUILD_TESTS "Enable testing" ON) ON?OFF? in yaml-cpp

When I compile yaml-cpp, the "undefined reference ..." occurs.

undefined reference to `testing::internal::EqFailure(char const*, char const*, std::__cxx11::basic_string

......

undefined reference to`testing::internal::GetBoolAssertionFailureMessage[abi:cxx11]

However, when option(YAML_CPP_BUILD_TESTS "Enable testing" ON) is changed to option(YAML_CPP_BUILD_TESTS "Enable testing" OFF) in CmakeLists.txt, yaml-cpp can be compiled successfully.

The version of GCC is 9.5.0. It seems to be about the criterion of C++11. Can anyone explain the reason for this phenomenon?

Any answer is appreciated.