support for custom .props files
This patch introduces a target property for tuning of Visual Studio MSBuild files (i.e. Visual Studio generators):
VS_USER_PROPS_CXX allows changing the .user.props
file from AppData\Local
to any custom path. This allows in particular to add the .user.props files to version control in software.
The introduction of the VS10_CXX_* defines is preparation for the C# support I will be submitting bit by bit.