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

[Daniel Chen via llvm-commits]

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@@ -142,6 +142,8 @@ function (add_flangrt_library name)
   endif ()
   if (build_shared)
     add_library("${name_shared}" SHARED ${extra_args} ${ARG_ADDITIONAL_HEADERS} ${ARG_UNPARSED_ARGUMENTS})
+    find_package(Threads REQUIRED)
+    target_link_libraries(${name_shared} PUBLIC Threads::Threads)
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DanielCChen wrote:

The intention of this change is to link in `libpthread.a` when binding the `libflang_rt.runtime.so` using `clang++` (the build compiler). 

Flang driver does indeed already have `-lpthread` at the linking step. But seems `check-flang-rt` passed.

Any suggestion of alternative if it is an issue?

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