Allow defined variables as values

[Aerilius]

This patch allows definitions based on previous definitions:

#define NARWHAL 0
#define UNICORN 1
#define RHINOCEROS 2
#define ANIMAL UNICORN

Previously, the fourth definition's value would be a string "UNICORN" and a condition like:

#if ANIMAL == UNICORN

would evaluate as "UNICORN" == 1 which is false.