Makes visible XPG4 standard symbols and causes _POSIX_C_SOURCE
    to be set to 2 if it is not already defined with a value greater
    than 2.

    Notes:

    o  Where the ISO C Amendment 1 includes symbols not specified by
       XPG4, defining __STDC_VERSION__  == 199409 and _XOPEN_SOURCE
       (or _XOPEN_SOURCE_EXTENDED) selects both ISO C and XPG4 APIs.
       Conflicts that arise when compiling with both XPG4 and ISO C
       Amendment 1 resolve in favor of ISO C Amendment 1.

    o  Where XPG4 extends the ISO C Amendment 1, defining _XOPEN_
       SOURCE or _XOPEN_SOURCE_EXTENDED selects ISO C APIs as well as
       the XPG4 extensions available in the header file. This mode of
       compilation makes XPG4 extensions visible.

    Standard Selected: XPG4

    Other Standards Implied: XPG4, ISO POSIX-2, ISO POSIX-1, ANSI C