[llvm] [Flang] explicitly link the pthread library when building shared flang-rt. (PR #129956)

[Daniel Chen via llvm-commits]

DanielCChen wrote:

@Meinersbur 
I have made another change to remove the `REQUIRED` and added a check to only link the pthread library if it exists because not all platform (e.g WIN32) has POSIX thread library.
Could you please take another look?

https://github.com/llvm/llvm-project/pull/129956