Autoconf configuration for doxygen.

Still need to add the comments in the source code. Currently I've
seeded it with the libprotobuf-c files.  I've configured it
to make man pages and html pages.  Might not be ideal, but makes it easy
for me to check things (html is nicer, but man pages are handier for
remote servers).
Default doxygen-doc to not enabled.

Also, don't default dot support to on and emit something to say if
the dogygen-doc target is enabled.
Disable dot in Doxyfile.
configure.ac: tweak the ${has_doxygen} status display
Fix -Wcast-align warnings when compiled with clang