This tells Github to detect files having the extension .def as C++
files, which ensures that these files get syntax highlighted prpperly.
I'm not sure whether more files like these are present throughout the
repo. FWIW, it can also be configured to do other things, such as detect
generated, vendored files and exclude these from the Github stats.
See
https://github.com/github-linguist/linguist/blob/master/docs/overrides.md.
