* doc/Doxyfile.in

  - activate DOT_MULTI_TARGETS, it is default on since doxygen 1.5.9

The description for this option is:
# Set the DOT_MULTI_TARGETS tag to YES allow dot to generate multiple output
# files in one run (i.e. multiple -o and -T options on the command line). This
# makes dot run faster, but since only newer versions of dot (>1.8.10)
# support this, this feature is disabled by default.

Even oldstable has a newer dot version, so the debian package doxygen
activate it per default and we will follow this recommendation now.

debian/changelog

@@ -38,6 +38,7 @@ apt (0.7.22.3) unstable; urgency=low
       thanks Modestas Vainius for noticing! (Closes: #531157, LP: #411435)
   * doc/Doxyfile.in:
     - update file with doxygen 1.6.1 (current unstable)
+    - activate DOT_MULTI_TARGETS, it is default on since doxygen 1.5.9
 
   [ George Danchev ]
   * cmdline/apt-cache.cc:

doc/Doxyfile.in

@@ -1500,7 +1500,7 @@ DOT_TRANSPARENT        = NO
 # makes dot run faster, but since only newer versions of dot (>1.8.10)
 # support this, this feature is disabled by default.
 
-DOT_MULTI_TARGETS      = NO
+DOT_MULTI_TARGETS      = YES
 
 # If the GENERATE_LEGEND tag is set to YES (the default) Doxygen will
 # generate a legend page explaining the meaning of the various boxes and