doxygen: switch to svg graphics

With this change doxygen will create SVG graphics (instead of png) which
are zoomable due these are vector graphics.

Suggested-by: Bill Bennet <william.bennett@netcommwireless.com>

doc/Doxyfile.in

@@ -2244,7 +2244,7 @@ DIRECTORY_GRAPH        = YES
 # The default value is: png.
 # This tag requires that the tag HAVE_DOT is set to YES.
 
-DOT_IMAGE_FORMAT       = png
+DOT_IMAGE_FORMAT       = svg
 
 # If DOT_IMAGE_FORMAT is set to svg, then this option can be set to YES to
 # enable generation of interactive SVG images that allow zooming and panning.
@@ -2256,7 +2256,7 @@ DOT_IMAGE_FORMAT       = png
 # The default value is: NO.
 # This tag requires that the tag HAVE_DOT is set to YES.
 
-INTERACTIVE_SVG        = NO
+INTERACTIVE_SVG        = @HAVE_DOT@
 
 # The DOT_PATH tag can be used to specify the path where the dot tool can be
 # found. If left blank, it is assumed the dot tool can be found in the path.