VSI C defines the following macros that you can explicitly define
  (using the /DEFINE qualifier or the #define preprocessor directive)
  to control which VSI C RTL functions are declared in header files
  and to obtain standards conformance checking:

  _XOPEN_SOURCE_EXTENDED
  _XOPEN_SOURCE
  _POSIX_C_SOURCE
  _ANSI_C_SOURCE
  _VMS_V6_SOURCE
  _DECC_V4_SOURCE
  __BSD44_CURSES
  __VMS_CURSES
  _SOCKADDR_LEN