How to exclude folders/files from error-checking in Wolfram Workbench

I have subfolders in my project which contain version control files (GNU-RCS). Naturally some of the lines in these files aren’t properly formed WL code and as a result, errors are generated.

Does anyone know how to switch off validation on specific folders/files?

I’ve already set the Derived property on the folder in question but this seems to have no effect.