D alternative for C/C++ -Dfoo=42

[Cherry]

>
> I don't like it. I would prefer a CTFE-able counter part to 
> boost::program_options that generates thouse enums from a 
> config file.

That would be good.

I would have taken that approach for my current needs, but I do 
not see any way to have a fallback if the config file does not 
exist. I mean mixin import would just fail to compile if the 
configuration file is not present. I need an option to live with 
default values if the configuration file is not there. Something 
like "static if(fileExists!"config.d") mixin(import("config.d")) 
else ...".

Thoughts?

Regards
- Cherry