Add a `VS_USER_PROPS_CXX` target property to set the user props file of
the generated `.vcxproj` file to be something other than the default
`$(UserRootDir)\\Microsoft.Cpp.$(Platform).user.props`.
With VS generators for 2010 and above, passing a `.targets` file to
`target_link_libraries` is expected to generate content in the
`.vcxproj` file to import the targets file. Add a test to cover this.