Fix MSVC2012 build

1) Look for Windows Kit (Platform SDK) -- provides opengl3 for QT5.
2) Specify link with SAFESEH:NO if using MSVC2012, since older
directinput lib (dx8?) fails to link with this enabled.

Could not test (only have MSVC2012 and qt5 installed at this point) but seems to fix build problems on my dev machine.