File polly-pthread.patch of Package llvm9

Since libPolly uses std::call_once it needs to be linked against
libpthread.  Due to our use of --as-needed we need to make sure it is
linked after libPolly.

Index: llvm-9.0.1.src/polly-9.0.1.src/lib/CMakeLists.txt
===================================================================
--- llvm-9.0.1.src.orig/polly-9.0.1.src/lib/CMakeLists.txt
+++ llvm-9.0.1.src/polly-9.0.1.src/lib/CMakeLists.txt
@@ -79,6 +79,7 @@ set_target_properties(PollyCore PROPERTI
 add_polly_library(Polly $<TARGET_OBJECTS:PollyCore>)
 target_link_libraries(Polly PRIVATE
   ${ISL_TARGET}
+  ${LLVM_PTHREAD_LIB}
 )
 
 # Additional dependencies for Polly-ACC.