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.

debian/rules

@@ -65,7 +65,8 @@ override_dh_auto_configure:
 	make clean
 	dh_auto_configure -- $(PRO2SETUP) $(LTOSETUP)	\
 	  --disable-devel --enable-optimizations	\
-	  --disable-static PYTHON=/usr/bin/$(PYDEFAULT)
+	  --disable-static --enable-doxygen \
+          PYTHON=/usr/bin/$(PYDEFAULT)
 	$(FLTOWORKAROUND)
 override_dh_auto_install: fix-js
 	dh_auto_install --destdir=$(CURDIR)/debian/tmp