Add substpdf target

Akin to substhtml, so that we can build PDF documents without
overwriting the upstream-provided versions and causing debian/rules clean
to not return to the original state.
merge patched into master
Build path-substituted documentation

Get actual paths, and also build outside of the source tree.
Update doc_base entries

The number and kind of documents has changed.
Synchronize .symbols files with upstream's exports

Upstream has removed some symbols without bumping the soname;
supposedly they are nonfunctional and thus should not have been used.
Update clean target

The new doc build produces some python droppings, clean them up.