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

[via llvm-commits]

jeremyd2019 wrote:

also, `llvm-config --cppflags` contains it

Perhaps it needs to explicitly add `LLVM_DEFINITONS` to `add_definitions` and `CMAKE_REQUIRED_DEFINITIONS`?

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