Stable release 1.12.13.

This contains improvements to the build process, the
Guile/Scheme binding ("SCM") and a new experimental "labels" sub-command. Let's
start with that last one.

* Labels

There's a new, experimental feature: *labels*. Labels are similar to the
existing "tags", but better integrated with both mu and mu4e,

You can search for message with a given label using 'label:', e.g
  $ mu find label:tupperware

Or add/remove labels, e.g., remove the label "planet" and add the label
"dwarf-planet" to all messages that have "pluto" in their subject:

  $ mu labels update "subject:pluto" --labels -planet,+dwarf-planet

Clear all labels from messages with the label "boring":

  $ mu labels clear "label:boring"

Labels are only stored in mu's database, and do not write to the message file
themselves. I.e., when you remove your database, your labels are gone. However,
you can use the `mu labels export` and `mu labels import` to save/restore them;
see the mu-labels(1) man-page for details.

In mu4e, you can do the same using `mu4e-headers-mark-for-label` (bound to `l`)
and clearing with `mu4e-headers-mark-for-unlabel` (bound to 'L').

I.e., mark some messages (or the current one by default), and in the
headers-buffer type
   l -planet +dwarf-planet RET
This works similar to the other marks.

There's autocompletion as well, both for search (label:) and marking (after +/-)

It's experimental, but it seems quite useful so far. See the man-page for
further details.

* SCM

The SCM/Guile bindings have been improved substantially; some common operations
are much faster (algorithmically). Also, the "glue" was added to interact with
Emacs and the "geiser" package, either with or without mu4e.

For the "plain Emacs" integraticon, see the Mu-SCM documentation, section
`Hooking up with GNU/Emacs and Geiser'.

For the mu4e integration, see the chapter `Using mu's SCM/Guile integration' in
the mu4e reference manual; or check the `mu4e-mu-scm-server' configuration
variable and the `mu4e-mu-scm-repl' command.

Future version will add more, and start answering questions like "Why would I
even want SCM bindings?!" :-)

* Building

- by default, tests are built 'lazily', i.e., only when running tests;
  this speeds up the build considerably for people that don't want to run
  tests (there's also -Btest=disabled of course)

- when available/found, mu now (by default) uses the system versions of CLI11
  and libfmt, rather than mu's "vendored" versions.

  You can influence where the build system (i.e., meson) looks through the
  `PKG_CONFIG_PATH` environment variable (see the pkg-config/pkgconf man-pages),
  and you can disable
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