Build support for Visual Studio 2013

These are the changes I had to make in order to get the solution to build in Windows 8.1/Visual Studio 2013 following the instructions at http://synergy-project.org/wiki/Compiling.

  
In Windows sdk 7.0A (corresponding to vs2010) Pbt.h file declares PBT_ constants and WM_POWERBROADCAST message.
All those constants are also defined in WinUser.h, so I think this include is useless if windows.h is included.