VMS Help CC, Language topics, Predefined Macros, RTL Standards Macros  *Conan The Librarian (sorry for the slow response - running on an old VAX)

  Compaq C defines the following macros that you can explicitly
  define (using the /DEFINE qualifier or the #define preprocessor
  directive) to control which Compaq 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