diff --git a/src/auto_timers_construction.cpp b/src/auto_timers_construction.cpp
index b23b546..db395e9 100644
--- a/src/auto_timers_construction.cpp
+++ b/src/auto_timers_construction.cpp
@@ -18,7 +18,9 @@
 
 // define BOOST_TIMER_SOURCE so that <boost/timer/config.hpp> knows
 // the library is being built (possibly exporting rather than importing code)
-#define BOOST_TIMER_SOURCE 
+#ifndef BOOST_TIMER_SOURCE
+# define BOOST_TIMER_SOURCE
+#endif
 
 #include <boost/timer/timer.hpp>
 #include <iostream>
diff --git a/src/cpu_timer.cpp b/src/cpu_timer.cpp
index 9443bc9..6b0e449 100644
--- a/src/cpu_timer.cpp
+++ b/src/cpu_timer.cpp
@@ -11,7 +11,9 @@
 
 // define BOOST_TIMER_SOURCE so that <boost/timer/config.hpp> knows
 // the library is being built (possibly exporting rather than importing code)
-#define BOOST_TIMER_SOURCE
+#ifndef BOOST_TIMER_SOURCE
+# define BOOST_TIMER_SOURCE
+#endif
 
 #include <boost/timer/timer.hpp>
 #include <boost/io/ios_state.hpp>