Avoid having text syntax-property changes mark buffer as modified

Old Emacsen don't have `with-silent-modifications', so copy the
definition of that in from subr.el for them.
Merge branch 'devel' containing 0.7 release candidate.

Among other things, this adds the gnuplot-context library for
context-sensitive completion and help lookup, and removes the ancient
info-look files that were causing problems (see issue #16). It also
changes the build process a bit (not that it matters when everyone
gets things from github or melpa these days anyway...)