Index: lib/Fuzzer/CMakeLists.txt
===================================================================
--- lib/Fuzzer/CMakeLists.txt
+++ lib/Fuzzer/CMakeLists.txt
@@ -1,3 +1,17 @@
+include(CheckCXXSourceCompiles)
+
+CHECK_CXX_SOURCE_COMPILES("
+    static thread_local int blah;
+    int main() {
+      return 0;
+    }
+    " HAS_THREAD_LOCAL)
+
+configure_file(
+    "${PROJECT_SOURCE_DIR}/lib/Fuzzer/FuzzerConfig.h.in"
+    "${PROJECT_BINARY_DIR}/lib/Fuzzer/FuzzerConfig.h"
+    )
+
 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")
Index: lib/Fuzzer/FuzzerConfig.h.in
===================================================================
--- /dev/null
+++ lib/Fuzzer/FuzzerConfig.h.in
@@ -0,0 +1 @@
+#define HAS_THREAD_LOCAL @HAS_THREAD_LOCAL@
Index: lib/Fuzzer/FuzzerInternal.h
===================================================================
--- lib/Fuzzer/FuzzerInternal.h
+++ lib/Fuzzer/FuzzerInternal.h
@@ -12,6 +12,7 @@
 #ifndef LLVM_FUZZER_INTERNAL_H
 #define LLVM_FUZZER_INTERNAL_H
 
+#include "FuzzerConfig.h"
 #include "FuzzerDefs.h"
 #include "FuzzerExtFunctions.h"
 #include "FuzzerInterface.h"
@@ -142,7 +143,11 @@
   size_t MaxMutationLen = 0;
 
   // Need to know our own thread.
+#if (HAS_THREAD_LOCAL == 1)
   static thread_local bool IsMyThread;
+#else
+  static __thread bool IsMyThread;
+#endif
 };
 
 }; // namespace fuzzer
Index: lib/Fuzzer/FuzzerLoop.cpp
===================================================================
--- lib/Fuzzer/FuzzerLoop.cpp
+++ lib/Fuzzer/FuzzerLoop.cpp
@@ -9,6 +9,7 @@
 // Fuzzer's main loop.
 //===----------------------------------------------------------------------===//
 
+#include "FuzzerConfig.h"
 #include "FuzzerCorpus.h"
 #include "FuzzerInternal.h"
 #include "FuzzerIO.h"
@@ -41,7 +42,11 @@
 namespace fuzzer {
 static const size_t kMaxUnitSizeToPrint = 256;
 
+#if (HAS_THREAD_LOCAL == 1)
 thread_local bool Fuzzer::IsMyThread;
+#else
+__thread bool Fuzzer::IsMyThread
+#endif
 
 SharedMemoryRegion SMR;
 
Index: lib/Fuzzer/build.sh
===================================================================
--- lib/Fuzzer/build.sh
+++ lib/Fuzzer/build.sh
@@ -1,6 +1,9 @@
 #!/bin/bash
 LIBFUZZER_SRC_DIR=$(dirname $0)
 CXX="${CXX:-clang}"
+cat <<EOF > $LIBFUZZER_SRC_DIR/FuzzerConfig.h
+#define HAS_THREAD_LOCAL 1
+EOF
 for f in $LIBFUZZER_SRC_DIR/*.cpp; do
   $CXX -g -O2 -fno-omit-frame-pointer -std=c++11 $f -c &
 done
@@ -8,4 +11,4 @@
 rm -f libFuzzer.a
 ar ru libFuzzer.a Fuzzer*.o
 rm -f Fuzzer*.o
-
+rm -f $LIBFUZZER_SRC_DIR/FuzzerConfig.h