Qt: Check for graphviz and set HAVE_DOT correctly

* configure.ac: Check for graphviz and define HAVE_DOT.
* lang/qt/doc/Doxyfile.in (HAVE_DOT): Use variable.

configure.ac

@@ -334,10 +334,20 @@ if test "$found" = "1"; then
             # This is not highlighted becase it's not really important.
             then AC_MSG_WARN([Doxygen not found - Qt binding doc will not be built.])
         fi
+        AC_CHECK_PROGS([GRAPHVIZ], [dot])
+        if test -z "$GRAPHVIZ";
+            then AC_MSG_WARN([Graphviz not found - Qt binding doc will not have diagrams.])
+        fi
     fi
 fi
 AM_CONDITIONAL([HAVE_DOXYGEN],
                [test -n "$DOXYGEN"])
+if test -n "$GRAPHVIZ"; then
+    HAVE_DOT="YES"
+else
+    HAVE_DOT="NO"
+fi
+AC_SUBST(HAVE_DOT)
 
 # Python bindings.
 LIST_MEMBER("python", $enabled_languages)

lang/qt/doc/Doxyfile.in

@@ -2087,7 +2087,7 @@ HIDE_UNDOC_RELATIONS   = YES
 # set to NO
 # The default value is: YES.
 
-HAVE_DOT               = YES
+HAVE_DOT               = @HAVE_DOT@
 
 # The DOT_NUM_THREADS specifies the number of dot invocations doxygen is allowed
 # to run in parallel. When set to 0 doxygen will base this on the number of