Doxygen only knows how to call dot with -Tpng, while using
-Tpng:gd produces pictures that are 10 times smaller.  Use a
simple wrapper around dot to simplify this.

* doc/dot.in: New file, that wrap the system's dot and replace
-Tpng by -Tpng:gd.
* doc/Makefile.am ($(srcdir)/stamp): Depend on dot.
* doc/Doxyfile.in: Update to 1.6.2.
(DOT_PATH): Set to @srcdir@ to use doc/dot instead of the
system's dot.
* configure.ac: Find the absolute path of dot, and generate
the doc/dot script.