Ninja fails to open the build log on Windows

[ldionne]

When we use a high value for the MEDIAN_OF argument, Ninja errors out on Windows. An example can be seen here.

This bug was already discussed in PR #109, ninja-build/ninja#1132, and in these comments. I'm creating a separate issue because the issue is already present (even before #109), it just didn't trigger, and I want to handle it separately so we can move on with #109.

[ldionne]

Ok, so I went ahead and disabled the Appveyor build temporarily in a3ecf8d. This will allow us to proceed with #109, and we can then try to find a solution for this issue.