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

[Michael Kruse via llvm-commits]

================
@@ -245,6 +245,8 @@ check_cxx_compiler_flag("-UTESTFLAG" FLANG_RT_SUPPORTS_UNDEFINE_FLAG)
 # Check whether -fno-lto is supported.
 check_cxx_compiler_flag(-fno-lto FLANG_RT_HAS_FNO_LTO_FLAG)
 
+# Search for pthread library
----------------
Meinersbur wrote:

Would be great to comment about why pthread is needed/what for, instead of what it is doing.

https://www.jackfranklin.co.uk/blog/code-comments-why-not-how/

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