Problem: mis-detection of threadsafe_static_init causes test failures

Solution: do not rely __cplusplus >= 201103L to detect whether the
compiler supports thread safe static initialisation, but check only
the proper feature preprocessor macro.
GCC introduced it in version 8, and Clang in version 6.

src/random.cpp

@@ -92,8 +92,7 @@ uint32_t zmq::generate_random ()
 //    configurable via config.h
 
 //  TODO this should probably be done via config.h
-#if __cplusplus >= 201103L                                                     \
-  || (defined(__cpp_threadsafe_static_init)                                    \
+#if (defined(__cpp_threadsafe_static_init)                                     \
      && __cpp_threadsafe_static_init >= 200806)                                \
   || (defined(_MSC_VER) && _MSC_VER >= 1900)
 #define ZMQ_HAVE_THREADSAFE_STATIC_LOCAL_INIT 1