Don't auto-create the subdirectories holding built documentation in a VPATH

Don't auto-create the subdirectories holding built documentation in a VPATH
build tree.  If we actually build the docs in the VPATH tree, those dirs
will get created then; but if they're present and empty, they capture the
vpathsearch searches in "make install", preventing installation of prebuilt
docs that might exist in the source tree.  Per bug #5595 from Dmtiriy Igrishin.
Fix based on idea from Peter Eisentraut.