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

[Martin Storsjö via llvm-commits]

mstorsjo wrote:

> so
> 
> ```cmake
> if (CYGWIN)
>   add_compile_definitions(_GNU_SOURCE)
>   list(APPEND CMAKE_REQUIRED_DEFINITIONS "-D_GNU_SOURCE")
> endif ()
> ```
> 
> right above, to go with the `SunOS` and `AIX` checks, and leave the `LLVM_USING_GLIBC` if alone?

Nah, I'd split the current `LLVM_USING_GLIBC` into two, one `if (LLVM_USING_GLIBC OR CYGWIN)` and one `if (LLVM_USING_CYGWIN)`. The comment above seems mostly relevant and on point for cygwin too, so we can keep the code in place where it is right now.

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