unifdef: -N <value> - eval constant #ifs with numeric values

This new option takes a numeric constant as an argument. Constant #ifs with
a numeric value>0 and <= the given value are processed (see '-k'). values
0 and > the given value are left untouched.

This can be used to get rid of the following code:
...
if 20140101	/* Change Date */
		/* new code */
		/* old code */
...