caneda: fix build with gcc6

pkgs/applications/science/electronics/caneda/default.nix

@@ -23,6 +23,10 @@ stdenv.mkDerivation rec {
 
   buildInputs = [ cmake qt4 libxml2 libxslt ];
 
+  patches = [
+    ./gcc6.patch
+  ];
+
   postInstall = ''
     mkdir $out/share/caneda/components
     cp -R ${srcComponents}/* $out/share/caneda/components

pkgs/applications/science/electronics/caneda/gcc6.patch

@@ -0,0 +1,13 @@
+diff --git c/src/cgraphicsscene.cpp i/src/cgraphicsscene.cpp
+index ac2929a..c399706 100644
+--- c/src/cgraphicsscene.cpp
++++ i/src/cgraphicsscene.cpp
+@@ -1436,7 +1436,7 @@ namespace Caneda
+             QPointF newPos = m_currentWiringWire->mapFromScene(pos);
+             QPointF refPos = m_currentWiringWire->port1()->pos();
+ 
+-            if( abs(refPos.x()-newPos.x()) > abs(refPos.y()-newPos.y()) ) {
++            if( (refPos.x()-newPos.x()) > (refPos.y()-newPos.y()) ) {
+                 m_currentWiringWire->movePort2(QPointF(newPos.x(), refPos.y()));
+             }
+             else {