Fix makefiles

* Remove unused `DMD` definition.
* Fix `LINKCMD` to use proper `dm` location.

---

The ICE is disappeared with other `dm.exe` but there are still make errors:
```
D:\D\dm\bin\linkold /SILENT /NOI /NOERROR  @..\slrNT
OPTLINK (R) for Win32  Release 8.00.5
Copyright (C) Digital Mars 1989-2009  All rights reserved.
http://www.digitalmars.com/ctg/optlink.html
LINK.def(0) : Error 2: File Not Found LINK.def
NMAKE : fatal error U1077: 'D:\D\dm\bin\linkold.EXE' : return code '0x1'
Stop.
NMAKE : fatal error U1077: '"C:\Program Files\Microsoft Visual Studio 9.0\VC\bin\nmake.EXE"' : return code '0x2'
Stop.
```