build: Build the bootstrap-graph.{png,eps,pdf} under $(top_srcdir).
Fixes <http://bugs.gnu.org/15640>.
Reported by Eric Bavier <ericbavier@gmail.com>.
* doc.am (EXTRA_DIST): Add doc/images/bootstrap-graph.pdf to the
distribution.
(pdf-local, info-local, ps-local): Prepend $(top_srcdir) to the image
file names.