backport fix from upstream to fix build errors with gcc 6

http://valgrind.10908.n7.nabble.com/Valgrind-r15773-in-trunk-configure-ac-drd-tests-std-thread-cpp-drd-tests-std-thread2-cpp-td56109.html

../../../valgrind-3.11.0/drd/tests/std_thread.cpp:30:3: error: '_Impl_base' is not a member of 'std::thread'
   std::thread::_Impl_base* __t = static_cast<std::thread::_Impl_base*>(__p);
   ^~~
../../../valgrind-3.11.0/drd/tests/std_thread.cpp:30:28: error: '__t' was not declared in this scope
   std::thread::_Impl_base* __t = static_cast<std::thread::_Impl_base*>(__p);
                            ^~~
../../../valgrind-3.11.0/drd/tests/std_thread.cpp:30:59: error: '_Impl_base' in 'class std::thread' does not name a type
   std::thread::_Impl_base* __t = static_cast<std::thread::_Impl_base*>(__p);
                                                           ^~~~~~~~~~

Signed-off-by: Khem Raj <raj.khem@gmail.com>
Upstream-Status: Backport

Index: configure.ac
===================================================================
--- a/configure.ac	(revision 15772)
+++ b/configure.ac	(revision 15773)
@@ -160,7 +160,7 @@
      icc-1[[3-9]].*)
 	AC_MSG_RESULT([ok (ICC version ${gcc_version})])
 	;;
-     notclang-[[3-9]].*|notclang-[[1-9][0-9]]*)
+     notclang-[[3-9]]|notclang-[[3-9]].*|notclang-[[1-9][0-9]]*)
 	AC_MSG_RESULT([ok (${gcc_version})])
 	;;
      clang-2.9|clang-[[3-9]].*|clang-[[1-9][0-9]]*)
Index: drd/tests/std_thread2.cpp
===================================================================
--- a/drd/tests/std_thread2.cpp	(revision 15772)
+++ b/drd/tests/std_thread2.cpp	(revision 15773)
@@ -26,6 +26,7 @@
   return 0;
 }
 
+#if defined(__GNUC__) && __GNUC__ -0 < 6
 //
 // From libstdc++-v3/src/c++11/thread.cc
 //
@@ -70,3 +71,4 @@
     }
   }
 }
+#endif
Index: drd/tests/std_thread.cpp
===================================================================
--- a/drd/tests/std_thread.cpp	(revision 15772)
+++ b/drd/tests/std_thread.cpp	(revision 15773)
@@ -21,6 +21,7 @@
   return 0;
 }
 
+#if defined(__GNUC__) && __GNUC__ -0 < 6
 //
 // From libstdc++-v3/src/c++11/thread.cc
 //
@@ -65,3 +66,4 @@
     }
   }
 }
+#endif