'Fix' for Ninja generator in Windows

Reference: http://www.cmake.org/Bug/view.php?id=15278
Until the bug is fixed in cmake, the generated files need to be fixed by
running 'sed' to replace '\' with '/'
Only 'fix' ninja in Windows

Also add a comment explaining the issue.
Merge pull request #84 from bogdanm/fix-ninja

'Fix' ninja build files on Windows