Yah it's not fun.
You might need to set MSVC_CC environment variable cause it
doesn't use the right format for VS path, depending on your
You could open a command prompt with the batch file running which
will add variables used by the script.
C:/Program Files (x86)/Microsoft Visual Studio
You should be able to build it now with just:
make -f win64.mak reldmd
When building druntime/phobos you need to pass the VCINSTALLDIR
environment variable, but it has an unexpected "/" at the end of
it so you need to remove it.
make -f win64.mak VCDIR="%VCINSTALLDIR:~0,-1%" DMD=../dmd/src/dmd
Didn't check if it works though I don't normally use batch/cmd.