refactor preprocessor macros into a function

Remove the "macros" global and instead use a function to construct a
list of macros.

prequeue.py

@@ -14,4 +14,4 @@ if __name__ == '__main__':
         dest = os.path.splitext(source)[0] + '.tx'
         sys.stdin  = open(source, 'r')
         sys.stdout = open(dest, 'w')
-        preprocessor.preprocess(preprocessor.macros)
+        preprocessor.preprocess(preprocessor.load_pokecrystal_macros())