VS: Convert include path to backslashes for VS >= 10

In VS <= 9 we already convert include paths to use backslashes
but the conversion was left out of the VS >= 10 generator.
Add the conversion now for consistency.  VS 11 and 12 now require
it because they mistakenly interpret forward-slash options that
happen to occur in paths:

 http://connect.microsoft.com/VisualStudio/feedback/details/778367
Merge topic 'vs10-include-backslashes' into next

c131107 VS: Convert include path to backslashes for VS >= 10