* doc/Doxyfile.in

* 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.