Make dot optional in generating documentation.

Different distributions of doxygen have different default
values of HAVE_DOT setting, so we need to pick a specific
setting to avoid 'missing dot' warnings on some platforms.

Doxygen uses it to generate inclusion graphs for our various
header files, which is somewhat useful, but not essential.
We therefore enable dot if it's present (usually through
the parent graphviz package) but disable it if it's not
available, silencing the warning, but not giving uniform
results.