how do I activate contracts for phobos functions in dmd

[Richard Palme via Digitalmars-d-learn]

My guess is that I have to build phobos with:

$make -f posix.mak BUILD=debug
but then what do I do next? I'm using Linux and I built 
dmd+phobos manually by following this guide: 
https://wiki.dlang.org/Building_under_Posix