Fix building ECM with CMake < 3.18
CheckLinkerFlags is available since CMake 3.18, for older versions assume ASAN linker flags are supported to restore the old behaviour and keep ECM buildable with older CMake versions.
CheckLinkerFlags is available since CMake 3.18, for older versions assume ASAN linker flags are supported to restore the old behaviour and keep ECM buildable with older CMake versions.