Build failure related to dependencies in a mixed solution

After converting a subset of projects from legacy project system to new in https://github.com/antlr/antlrcs/pull/74/commits/4b0ae50a1fcc01e1f149c0d79272fb69da0a423b, I found that the build failed due to a file access error until I turned off parallel builds (removed `/m`) in https://github.com/antlr/antlrcs/pull/74/commits/407a344523fa4bd1bf9492827156bc2dfd9b2982. I'm guessing this is caused by a failure to properly track dependencies when the build contains both legacy and new projects.

I was not able to reproduce this locally in Visual Studio, but the AppVeyor build was consistently failing with the `/m` flag.