DFLAGS ignored. How to get a dmd installation on windows that is 64 bit only

[Carl Sturtivant via Digitalmars-d-learn]

The docs for dmd for windows say that the DFLAGS environment 
variable's value will be appended to the dmd command line. I 
tried this with -m64 as the value and this is ignored.
http://dlang.org/dmd-windows.html#environment
More generally, is there a standard rearrangement of files and 
environment variables so that all compilation and linking is 64 
bit, and 32-bit stuff is simply not there?