MSVC: do not include windef.h in your code
MSVC: do not include windef.h in your code

See http://stackoverflow.com/questions/4845198/fatal-error-no-target-architecture-in-visual-studio
That's exactly the error we get with MSVC 2015:
>C:\Program Files (x86)\Microsoft SDKs\Windows\v7.0A\include\winnt.h(135): fatal error C1189: #error :  "No Target Architecture"

The good practice is to include <windows.h>, and it's also true for mingw.
(this change also fix a _STRUCT_NAME redefinition warning with mingw).

timfel/squeakvm,bencoman/pharo-vm,timfel/squeakvm,OpenSmalltalk/vm,bencoman/pharo-vm,OpenSmalltalk/vm,peteruhnak/pharo-vm,timfel/squeakvm,peteruhnak/pharo-vm,OpenSmalltalk/vm,peteruhnak/pharo-vm,bencoman/pharo-vm,peteruhnak/pharo-vm,timfel/squeakvm,peteruhnak/pharo-vm,bencoman/pharo-vm,bencoman/pharo-vm,bencoman/pharo-vm,bencoman/pharo-vm,peteruhnak/pharo-vm,OpenSmalltalk/vm,bencoman/pharo-vm,OpenSmalltalk/vm,peteruhnak/pharo-vm,OpenSmalltalk/vm,peteruhnak/pharo-vm,bencoman/pharo-vm,timfel/squeakvm,OpenSmalltalk/vm,timfel/squeakvm,OpenSmalltalk/vm,timfel/squeakvm,timfel/squeakvm