--store now has the same behavior as the 'store' directive: the store
must be a mount point. --unmounted-store allows store directories
that aren't mount points. As well as providing an escape hatch to the
previous behavior we need this for testing.

re #42

This can be disabled with new options to store and store-pattern.

fixes #42

fixes #46

fixes #47

(cherry picked from commit b298f64b)

...and ref it on the web. LyX is broken (at least locally, for this
input) in sid, and it's not actually that useful to have this stuff
lying around in the source code anyway.

fixes #43

Squashed commit of:

commit 71923796b625c48beb09c057272433da7fc8ded5
Author: Richard Kettlewell <rjk@greenend.org.uk>
Date:   Fri Feb 2 12:39:14 2018 +0000

    tests/issue43: we require bash

    This is possible to work around (see setup.sh for similar workarounds
    already) but far too annoying.

commit f46b5c1336607ddfddb7b720e9c985f1abec5798
Author: Richard Kettlewell <rjk@greenend.org.uk>
Date:   Tue Jan 30 22:07:01 2018 +0000

    pre-backup-hook can soft fail

re #41

Broken header files.

Also test that 'partial transfer' warning suppression works.

fixes #40

fixes #36

90s crypto, but what really matters is the order...

fixes #35

Document the situation for backups with macOS.

prune-age and min-backups have been deprecated since 3.0,
so I don’t feel bad about reducing their test coverage.

Fixes #37 (again).

Fixes #37

Adds support for CXXFLAGS_EXTRA in configure script.

We still install g++-6 even though we're going to build with Clang, to
dodge trusty's native broken c++ regex implementation.

Travis's ancient Ubuntu platform is a bit special.