The %IF family of directives (%IF, %ELSE, %ELIF, %ENDIF) is used
  to  conditionally  compile  specific  sections  of  source code.
  These directives are useful if you  need  to  compile  the  same
  source code for various configurations or environments.

    %IF compile-time-expression
    %THEN
        Pascal tokens ...
    [%ELIF compile-time-expression
     %THEN
         Pascal tokens ... ] ...
    [%ELSE
         Pascal tokens ... ]
    %ENDIF

  A %IF directive can have zero or more %ELIF parts  and  zero  or
  one %ELSE parts.

  %IF directives can be nested up to 32 deep.

  Note that skipped sections of source code must  still  be  valid
  VSI  Pascal  tokens.   The  skipped  tokens  are  not  processed
  semantically by the compiler.

  The compile-time expression for the %IF and %ELIF directives  is
  the  same  compile-time  expression that can be used anywhere in
  VSI Pascal.

  The /CONSTANT DCL qualifier can be used together  with  the  %IF
  directive to control the conditional processing from the command
  line.