Place where dot executable is found

In the current vhdl part of the code it is required that the dot executable is in the path. On in dot.cpp the possibility exists that the dot executable is found through the configuration setting DOT_PATH.
This patch makes this more consistent

src/vhdldocgen.cpp

@@ -167,7 +167,8 @@ static void createSVG()
     QRegExp ep("[\\s]");
     QCString vlargs="-Tsvg \""+ov+"\" "+dir ;
 
-    if (portable_system("dot",vlargs)!=0)
+    QCString dotExe   = Config_getString("DOT_PATH")+"dot";
+    if (portable_system(dotExe,vlargs)!=0)
     {
       err("could not create dot file");
     }
@@ -3884,8 +3885,9 @@ void FlowChart::createSVG()
   ov+="/flow_design.dot";
 
   QCString vlargs="-Tsvg "+ov+dir ;
+  QCString dotExe   = Config_getString("DOT_PATH")+"dot";
 
-  if (portable_system("dot",vlargs)!=0)
+  if (portable_system(dotExe,vlargs)!=0)
   {
     err("could not create dot file");
   }