add @#ifdef functionality to macroprocessor

Currently, we only support @#if, meaning that every macro variable must be predefined. For large mod files with many clauses where you may want to include/exclude portions easily, this becomes cumbersome because it requires you to define all macro used anywhere to be equal to zero at the beginning of the modfile.

It is also cumbersome when certain macro variables are only defined based on the existence of others, making the definition at the beginning of the mod file all that much longer.

See GPM6.mod to get the general idea.