DUSE_MYLIB12

[Dennis Ritchie via Digitalmars-d-learn]

Tell me, please, is it possible to set an arbitrary condition 
conditional compilation at the command prompt, type DUSE_MYLIB12 
and in the code as:

version(USE_MYLIB12) {
        .....
}

And I didn't like Any DeclarationBlock or Statement that is not 
compiled in still must be syntactically correct:
http://dlang.org/version.html

It may be that using an optional library code is correct, and 
without it not?