Script: Allow #define to reference other macros

Support for recursive macro definitions. Tested against VC++ with:
```
#define RED    GREEN
#define BLUE   456
#define GREEN  BLUE
#undef BLUE
#define BLUE  123
```
RED is 123 for both VC++ and AGS. Note: C# doesn't support #define constants, or I would've tested against that.

Also, I believe you ran into the problem this fixes, Ivan:
http://www.adventuregamestudio.co.uk/forums/index.php?topic=41144.0;nowap