need explicit reference to subdir to be created
Get the out-of-tree build of docs right - finally?

Trouble is that sphinx only takes one source dir, but when we build
out-of-tree, we need to both use sources from $(srcdir) as well as
$(builddir).

Fix this by hard-linking from $(srcdir) to $(builddir) and wait for
someone to come up with an out-of-tree build on different file systems.

Fixes #1479