Script: Allow #define to reference other macros

Support for recursive macro definitions. Tested against VC++ with:

 #define RED    GREEN
 #define GREEN  BLUE
 #define BLUE   456
 #undef BLUE
 #define BLUE  123

BLUE is 123 for both VC++ and AGS. Note: C# doesn't support #define
constants, or I would've tested against that.