Index: lib/Fuzzer/CMakeLists.txt
===================================================================
--- lib/Fuzzer/CMakeLists.txt
+++ lib/Fuzzer/CMakeLists.txt
@@ -1,3 +1,16 @@
+include(CheckCXXSourceCompiles)
+
+CHECK_CXX_SOURCE_COMPILES("
+    static thread_local234 int blah;
+    int main() {
+      return 0;
+    }
+    " HAS_THREAD_LOCAL)
+
+if ( NOT HAS_THREAD_LOCAL )
+    set (CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -Dthread_local=__thread" )
+endif()
+
 if( LLVM_USE_SANITIZE_COVERAGE )
   set(LIBFUZZER_FLAGS_BASE "${CMAKE_CXX_FLAGS}")