Add missing sphinx-build make dependencies

At this point I'm wondering whether we should talk about hacks when we
really add missing dependencies. In some cases it boils down to RST
being generated by programs we need to compile, what I call "regular"
targets here.

Makefile.am

@@ -56,6 +56,10 @@ LICENSE: all
 # before we try to run tests anywhere in the tree.
 check-recursive: all
 
+# XXX: This is the exact same hack since some parts of the documentation
+# are generated as regular targets but needed by the html special target.
+html-recursive: all
+
 cscope:
 	-rm -f cscope*
 	find . -name '*.[hcS]' > cscope.files