fix warning
Add support for winsdk8 (vs 11.0 / vs2012)

Some file paths were changed with visual studio 2012 (version 11.0),
and the new winsdk8:

- Traditional windows API are found under a "UM" (user-mode) directory.
- WinRT (new APIs) are found under a "WINRT" directory
- Others in "SHARED" directory

We import the UM headers for now. WinRT support should come probably with
its own toolset variant.
make logic more readable thanks to short circuit
Merge pull request #136 from uucidl/F-MsvcWinsdkFor2012

msvc-winsdk support for winsdk8 and visual studio 2012 (11.0)