cmake LIBURINGCXX_HAS_OPENAT2 examples

Codesire-Deng/co_context extern/liburingcxx/cmake/configure.cmake :33

check_cxx_source_runs(
    [====[
    #include <linux/openat2.h>
    int main(){ return 0; }
    ]====]
    LIBURINGCXX_HAS_OPENAT2
)