VMS Help CC, /UNDEFINE  *Conan The Librarian (sorry for the slow response - running on an old VAX)

     /UNDEFINE=(identifier[,...])
     /NOUNDEFINE (D)

  Performs the same function as the #undef preprocessor directive:
  it cancels a macro definition.

  The /UNDEFINE qualifier is useful for undefining the predefined
  Compaq C preprocessor constants.  For example, if you use a
  preprocessor constant (such as vaxc, VAXC, VAX11C, or vms) to
  conditionally compile segments of code specific to Compaq C for
  OpenVMS Systems, you can undefine constants to see how the portable
  sections of your program execute.  For example:

  /UNDEFINE="vaxc"

  When both /DEFINE and /UNDEFINE are present on the CC command line,
  /DEFINE is evaluated before /UNDEFINE.

  The default is /NOUNDEFINE.