Related to https://github.com/mtytel/cursynth/issues/9

--- a/mopo/src/wave.h	2014-06-25 14:10:57.827369578 -0500
+++ b/mopo/src/wave.h	2014-06-25 14:08:15.795376354 -0500
@@ -19,6 +19,7 @@
 #define WAVE_H
 
 #include "mopo.h"
+#include <cstdlib>
 #include <cmath>
 
 #define LOOKUP_SIZE 2048
