VS2012: use v110_xp toolset.
VS2012: use v110_xp toolset.

This ensures that WinSparkle.dll is still usable on Windows XP, for
users of VS2012 that need to ship for XP.

CaptainJackJJ/SmpUpdater,bristol-instruments/winsparkle,Herve-M/winsparkle,dsuess/winsparkle,alamaison/winsparkle,artillery/winsparkle,CaptainJackJJ/SmpUpdater,dsuess/winsparkle,alamaison/winsparkle,nickdademo/winsparkle,drizt/winsparkle,artillery/winsparkle,drizt/winsparkle,bristol-instruments/winsparkle,nickdademo/winsparkle,Herve-M/winsparkle