BLD: Fix msvc build error C1189: "No Target Architecture"

Fixes the following compiler error on Windows when building with msvc10 and SDK 7.1 for 64 bit:
```
_imagingcms.c
X:\WinSDK71\include\winnt.h(135) : fatal error C1189: #error :  "No Target Architecture"
error: command '"C:\Program Files (x86)\Microsoft Visual Studio 10.0\VC\BIN\amd64\cl.exe"' failed with exit status 2
```