To build DMD as a 64-bit binary: make -flinux.mak MODEL=-m64 To build Phobos or Druntime as 64-bit: make -fposix.mak MODEL=64 These are not consistent and should be.
Worth noting, you don't need to build dmd as a 64 bit binary to have it generate 64 bit output. I do agree that consistency would be good. While fixing inconsistencies, there's a similar one for building phobos between the posix and win32 make files. posix.mak uses DRUNTIME_PATH and win32.mak uses DRUNTIME.
Fixed with the new posix.mak makefile.