fix problem with thread local and shared ptr on OSX

Utility/Utility/use_atomic.h

+#ifndef CLHEP_USE_ATOMIC_GUARD_H
+#define CLHEP_USE_ATOMIC_GUARD_H
+
+// ======================================================================
+//
+// Use std::atomic when the compiler declares it uses the C++11 standard
+//
+// ======================================================================
+
+#if __cplusplus >= 201103L
+
+  #if __GNUC__ > 4 || (__GNUC__ == 4 && __GNUC_MINOR__ > 7)
+    #include <atomic>
+    #define CLHEP_USE_ATOMIC
+  #elif __clang__
+    #if __has_feature(c_atomic)
+      #include <atomic>
+      #define CLHEP_USE_ATOMIC
+    #endif
+  #endif
+
+#endif
+
+#endif