[llvm] fix zstd_shared detection on mingw (PR #139945)

[Martin Storsjö via llvm-commits]

mstorsjo wrote:

> I updated the github message, so you can just take that text when merging (I don't have merge rights)

Thanks!

> The zstd_shared autodetection was broken for non-MSVC (mingw or clang) compiled LLVM

I'd like to nitpick the wording here, "mingw or clang" feels like a bit of an oxymoron (or something...) here. "Mingw" is the name of the environment, not a name that specifically means GCC; and clang can either be in mingw mode or in msvc mode. And clang in msvc mode has the same naming style/behaviour as regular MSVC.

Anyway, by just replacing the parenthesis with "(mingw)" it sounds fine to me. I'll approve the patch and I can update the PR description.

https://github.com/llvm/llvm-project/pull/139945