Issue on non-case sensitive filesystems: `sources` and `SOURCES`

[marctrem]

./sources and ./SOURCES do not play well on non-case sensitive filesystems (such as macOS's out-of-the-box configuration).

This also means that on macOS, a checkout cannot be "not dirty".

Would it be possible to consider using filenames which don't clash on non-case sensitive filesystems?

Thank you!

[cr-tk]

@marctrem I think this is a duplicate of the old #231.