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

This patch is to explicitly link the pthread library when building
shared flang-rt.
On AIX, for example, it needs to link in `libpthread.a` explicitly in
order to resolve the references to those `pthread_*` functions in
`include/flang-rt/runtime/lock.h`

Author: DanielCChen

flang-rt/CMakeLists.txt

@@ -245,6 +245,10 @@ 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)
 
+# Different platform may have different name for the POSIX thread library.
+# For example, libpthread.a on AIX. Search for it as it is needed when
+# building the shared flang_rt.runtime.so.
+find_package(Threads)
 
 # function checks
 find_package(Backtrace)

flang-rt/cmake/modules/AddFlangRT.cmake

@@ -142,6 +142,9 @@ function (add_flangrt_library name)
   endif ()
   if (build_shared)
     add_library("${name_shared}" SHARED ${extra_args} ${ARG_ADDITIONAL_HEADERS} ${ARG_UNPARSED_ARGUMENTS})
+    if (Threads_FOUND) 
+      target_link_libraries(${name_shared} PUBLIC Threads::Threads)
+    endif ()
   endif ()
 
   if (libtargets)