Cannot use ObjectFileName with a simple slash

[GoogleCodeExporter]

What steps will reproduce the problem?
1. Create a gyp project with
          'msbuild_settings': {
            'ClCompile': {
              'ObjectFileName': '$(IntDir)/%(RelativeDir)',
            },
2. Generate the visual studio project
3.

Expected output:
ObjectFileName in the properties should be '$(IntDir)/%(RelativeDir)' but it is 
just '$(IntDir)%(RelativeDir)'.
The slash gets removed,. Same behavior with '$(IntDir)\%(RelativeDir)'.
The slash remains with an extra space, or any character in between '$(IntDir) 
/%(RelativeDir)'

My gyp version is probably a bit old, not sure what version it is.
Operating system: Windows 8.1

Original issue reported on code.google.com by maxime.v...@serato.com on 14 May 2015 at 12:12