dmd can't build gtk x64

[FoxyBrown via Digitalmars-d-learn]

Error: can't run 
'C:\VS\VC\Tools\MSVC\14.10.25017\bin\HostX64\x64', check PATH

It is trying to run some x64.exe/com/bat file... why? What is 
this file?

simply doing dmd -m64 build.d

Works fine for x86.