Add some easy way of declaring preproccesor defines

[Adanteh]

I use quite a bit of custom defines, and it would be nice if you could easily add some detection for this.
Think along the lines of GVAR, EGVAR like CBA and ACE use, but then also custom ones like CVAR, MVAR (All my mods have a 'core' and 'main' module) that I'd like to highlight like that.