Merge branch 'optional_libunwind' into 'master'

Allow to opt-out from using libunwind.

See merge request !109

(cherry picked from commit 8081f466)

a1377f34 Allow to opt-out from using libunwind.
b120525f Turn the 'libunwind' build option into a boolean.
6 jobs for geary-0.13 in 23 minutes and 45 seconds (queued for 1 second)
Status Job ID Name Coverage
  Build
passed #221379
fedora

00:08:16

passed #221380
ubuntu

00:13:01

 
  Test
passed #221381
deb-package

00:10:43

failed #221382
flatpak

00:00:52

 
  Deploy
skipped #221383
review
skipped #221384
allowed to fail manual
stop_review
 
Name Stage Failure
failed
flatpak Test
$ flatpak-builder --stop-at=${FLATPAK_MODULE} app ${MANIFEST_PATH}
error: org.gnome.Sdk/x86_64/3.30 not installed
Failed to init: Unable to find sdk org.gnome.Sdk version 3.30
Uploading artifacts...
WARNING: geary-dev.flatpak: no matching files
WARNING: _build/meson-logs/meson-log.txt: no matching files
WARNING: _build/meson-logs/testlog.txt: no matching files
ERROR: No files to upload
ERROR: Job failed: exit code 1