Feature-test macros provide a means for writing portable
    programs. They ensure that the C RTL symbolic names used
    by a program do not clash with the symbolic names supplied by the
    implementation.

    The C RTL header files are coded to support the use of
    a number of feature-test macros. When an application defines a
    feature-test macro, the C RTL header files supply the
    symbols and prototypes defined by that feature-test macro and
    nothing else. If a program does not define such a macro, the
    C RTL header files define symbols without restriction.

    The feature-test macros supported by the C RTL fall into
    the following broad categories for controlling the visibility of
    symbols in header files according to the following:

    o  Standards

    o  Multiple-version support

    o  Compatibility