S_IFMT is a POSIX extension, so it might require some other macros be defined, e.g. when compiled with-std=c11 we can pair it with_XOPEN_SOURCE=700,_DEFAULT_SOURCE,_GNU_SOURCE, ...

https://stackoverflow.com/questions/28547271/s-ifmt-and-s-ifreg-undefined-with-std-c11-or-std-gnu11

As to why-std=c11 was chosen, well, I don't know :)