doc: implement --enable-doxygen and do not distribute the doc

Fixes #299.

* configure.ac, doc/Makefile.am: Adjust.
* NEWS, HACKING, README: Document the change.
* doc/dot.in: Delete, not used anymore.
* doc/Doxyfile.in: Adjust to not look for dot.
* debian/rules: Use --enable-doxygen.

NEWS

@@ -1,5 +1,13 @@
 New in spot 2.4.2.dev (not yet released)
 
+  Build:
+
+  - We no longuer distribute the doxygen-generated documentation in
+    the tarball of Spot to save space.  This documentation is still
+    available online at https://spot.lrde.epita.fr/doxygen/.  If you
+    want to build a local copy you can configure Spot with
+    --enable-doxygen, or simply run "cd doc && make doc".
+
   Tools:
 
   - genltl learned to generate a new family of formulas, taken from