Fixed build when std::atomic is not available
diff --git a/src/main/include/log4cxx/helpers/mutex.h b/src/main/include/log4cxx/helpers/mutex.h
index aecc6bd..e4b0411 100644
--- a/src/main/include/log4cxx/helpers/mutex.h
+++ b/src/main/include/log4cxx/helpers/mutex.h
@@ -22,7 +22,9 @@
 
 #include <apr_portable.h>
 
+#if defined(RW_MUTEX)
 #include <atomic>
+#endif
 
 extern "C" {
    struct apr_thread_mutex_t;