Doxygen supports the creation of DOT graphics, but this depends on the
existence of the 'dot' binary on the build host. Adding a check for dot
and disable the support within the Doxygen file for DOT graphics if dot
can't be found.
To install the doxygen output, which is generated within doc/html/,
adding the needed checks for the doxygen binary to configure.ac and also
adding the needed doc/Makefile.am file.
Finally tuning doc/Doxyfile.in to let the autotools write the needed
informations into this file while generating the finally created
doc/Doxyfile.
Carsten Schoenert