Enhancement: Allow file exclusion where file globs are supported

[TheDrawingCoder-Gamer]

When adding extra source files, I noticed that it includes files I don't want. However, to get around this I would have to write much more patterns then are needed. Allowing glob pattern exclusion (similar to !file syntax in .gitignore files) would make excluding files much easier. So e.g.

extra-source-files:
- c/**
- "!c/pamixer/build/**"

would include all files under c except c/pamixer/build