Add ELSEIF compiler directive

[Christian-W. Budde]

I wonder how much work it is to support the ELSEIF compiler directive. This would improve taking care for various exclusive defines. For example:

{$IF DEFINED('ChoiceA')}
  // do something
{$ELSEIF DEFINED('ChoiceB')}
  // do something
{$ELSEIF DEFINED('ChoiceC')}
  // do something
{$ENDIF}

The above is just an example for compile time support of an exclusive switch. In this case it's always either ChoiceA or ChoiceB or ...

Without such a compiler directive the code gets either unaccurate:

{$IF DEFINED('ChoiceA')}
  // do something
{$ENDIF}
{$IF DEFINED('ChoiceB')}
  // do something
{$ENDIF}
{$IF DEFINED('ChoiceC')}
  // do something
{$ENDIF}

or rather ugly:

{$IF DEFINED('ChoiceA')}
  // do something
{$ELSE}
{$IF DEFINED('ChoiceB')}
  // do something
{$ELSE}
{$IF DEFINED('ChoiceC')}
  // do something
{$ENDIF}
{$ENDIF}
{$ENDIF}

It's not high priority, but since this is quite independent, it might be quite easy to add.

[Christian-W. Budde]

Code formatting

[Eric Grange]

Had a quick try with the code below, it's a bit tricky than I first thought, and some more tests with nested conditionals and cascading ELSEIF would probably be warranted before an implementation can be trusted.

var abc : String;

{$IF Declared('abc')}
PrintLn('abc declared');
{$ELSEIF Declared('xyz')}
PrintLn('bug 1');
{$ELSE}
PrintLn('bug 2');
{$ENDIF}

{$IF Declared('xyz')}
PrintLn('bug 3');
{$ELSEIF Declared('abc')}
PrintLn('abc declared indeed');
{$ELSE}
PrintLn('bug 4');
{$ENDIF}

{$IF Declared('xyz')}
PrintLn('bug 5');
{$ELSEIF not Declared('abc')}
PrintLn('bug 6');
{$ELSE}
PrintLn('ok');
{$ENDIF}