Index: llvm/trunk/lib/Fuzzer/CMakeLists.txt
===================================================================
--- llvm/trunk/lib/Fuzzer/CMakeLists.txt
+++ llvm/trunk/lib/Fuzzer/CMakeLists.txt
@@ -1,3 +1,16 @@
+include(CheckCXXSourceCompiles)
+
+CHECK_CXX_SOURCE_COMPILES("
+    static thread_local int blah;
+    int main() {
+      return 0;
+    }
+    " HAS_THREAD_LOCAL)
+
+if( NOT HAS_THREAD_LOCAL )
+    set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -Dthread_local=__thread")
+endif()
+
 set(LIBFUZZER_FLAGS_BASE "${CMAKE_CXX_FLAGS}")
 # Disable the coverage and sanitizer instrumentation for the fuzzer itself.
 set(CMAKE_CXX_FLAGS "${LIBFUZZER_FLAGS_BASE} -fno-sanitize-coverage=trace-pc-guard,edge,trace-cmp,indirect-calls,8bit-counters -Werror")