[llvm] [LLVM][Cygwin] Define _GNU_SOURCE on Cygwin as well. (PR #138329)

[Martin Storsjö via llvm-commits]

mstorsjo wrote:

> cygwin's packaging builds clang standalone, and in that configuration it seems that `_GNU_SOURCE` is not defined despite this change. Any thoughts?

Hmm. I don't really have any experience with building clang standalone in that way; does the clang source tree have any form of local corresponding code that sets `_GNU_SOURCE` for glibc there? I presume you've tested this after rebuilding llvm with these changes, so it's not carried on via some of the llvm cmake files? (I'm not sure how the standalone builds work if they inherit things from such files, or if it configures things up from scratch.)

https://github.com/llvm/llvm-project/pull/138329