[geary/wip/update-libunwind-build-option] Flip the sense of the libunwind build option

[Michael Gratton <mjog src gnome org>]

commit ee86739373a2315d1a51ca4df71682f25a629735
Author: Michael Gratton <mike vee net>
Date:   Thu Feb 21 12:00:17 2019 +1100

    Flip the sense of the libunwind build option
    
    Updates !109, see also #238

 meson.build       | 8 ++++++--
 meson_options.txt | 2 +-
 2 files changed, 7 insertions(+), 3 deletions(-)
---
diff --git a/meson.build b/meson.build
index 0e6d0143..ee4a5f9d 100644
--- a/meson.build
+++ b/meson.build
@@ -70,8 +70,12 @@ libmath = cc.find_library('m')
 libnotify = dependency('libnotify', version: '>= 0.7.5')
 libsecret = dependency('libsecret-1', version: '>= 0.11')
 libsoup = dependency('libsoup-2.4', version: '>= 2.48')
-libunwind_dep = dependency('libunwind', version: '>= 1.1', required: get_option('libunwind'))
-libunwind_generic_dep = dependency('libunwind-generic', version: '>= 1.1', required: get_option('libunwind'))
+libunwind_dep = dependency(
+  'libunwind', version: '>= 1.1', required: not get_option('libunwind_optional')
+)
+libunwind_generic_dep = dependency(
+  'libunwind-generic', version: '>= 1.1', required: not get_option('libunwind_optional')
+)
 libxml = dependency('libxml-2.0', version: '>= 2.7.8')
 posix = valac.find_library('posix')
 webkit2gtk_web_extension = dependency('webkit2gtk-web-extension-4.0', version: '>=' + target_webkit)
diff --git a/meson_options.txt b/meson_options.txt
index 63bbce7e..2bd04e4a 100644
--- a/meson_options.txt
+++ b/meson_options.txt
@@ -4,4 +4,4 @@ option('poodle', type: 'boolean', value: true, description: 'Whether to apply th
 option('ref_tracking', type: 'boolean', value: false, description: 'Whether to use explicit reference 
tracking.')
 option('iso_639_xml', type: 'string', value: '', description: 'Full path to the ISO 639 XML file.')
 option('iso_3166_xml', type: 'string', value: '', description: 'Full path to the ISO 3166 XML file.')
-option('libunwind', type: 'boolean', value: true, description: 'Whether to depend on libunwind.')
+option('libunwind_optional', type: 'boolean', value: false, description: 'Determines if libunwind is 
required.')