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

  The following predefined macros are defined as 1 if the
  corresponding compiler mode is selected (Otherwise, they are
  undefined):

  __DECC_MODE_STRICT     !  /STANDARD=ANSI89
  __DECC_MODE_RELAXED    !  /STANDARD=RELAXED_ANSI89
  __DECC_MODE_VAXC       !  /STANDARD=VAXC
  __DECC_MODE_COMMON     !  /STANDARD=COMMON
  __STDC__               !  /STANDARD=ANSI89, /STANDARD=RELAXED_ANSI89
  __STDC_VERSION__       !  /STANDARD=ISOC94
  __MS                   !  /STANDARD=MS