You can define a feature-test macro to select each standard. You
    can do this either with a #define preprocessor directive in your
    C source before the inclusion of any header file, or with the
    /DEFINE qualifier on the CC command line.

    Features not defined by one of the previously named standards are
    considered VSI C extensions and are selected by not defining
    any standards-related, feature-test macros.

    If you do not explicitly define feature test macros to control
    header file definitions, you implicitly include all defined
    symbols as well as VSI C extensions.