Remove REQUIRED as not all the platform has pthread library.

Author: DanielCChen

flang-rt/CMakeLists.txt

@@ -248,7 +248,7 @@ 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 REQUIRED)
+find_package(Threads)
 
 # function checks
 find_package(Backtrace)

flang-rt/cmake/modules/AddFlangRT.cmake

@@ -142,7 +142,9 @@ function (add_flangrt_library name)
   endif ()
   if (build_shared)
     add_library("${name_shared}" SHARED ${extra_args} ${ARG_ADDITIONAL_HEADERS} ${ARG_UNPARSED_ARGUMENTS})
-    target_link_libraries(${name_shared} PUBLIC Threads::Threads)
+    if (Threads_FOUND) 
+      target_link_libraries(${name_shared} PUBLIC Threads::Threads)
+    endif ()
   endif ()
 
   if (libtargets)