Upstream bug:
https://sourceforge.net/tracker/index.php?func=detail&aid=2447869&group_id=86211&atid=578826

Index: qjackctl-0.3.4/src/qjackctlPatchbayRack.cpp
===================================================================
--- qjackctl-0.3.4.orig/src/qjackctlPatchbayRack.cpp
+++ qjackctl-0.3.4/src/qjackctlPatchbayRack.cpp
@@ -30,6 +30,7 @@
 //
 
 #include <QHash>
+#include <cstdlib>
 
 class qjackctlPatchbaySnapshot
 {