build: Add an option to build against libmalcontent from the system

Rather than building it again; this is the second half of resolving the
dependency cycle from the previous commit.

Signed-off-by: Philip Withnall <withnall@endlessm.com>

Fixes: #16

meson.build

@@ -126,7 +126,14 @@ test_env = [
 ]
 
 subdir('accounts-service')
+if not get_option('use_system_libmalcontent')
   subdir('libmalcontent')
+else
+  libmalcontent_api_version = '0'
+  libmalcontent_dep = dependency('malcontent-' + libmalcontent_api_version, version: meson.project_version())
+  libmalcontent_gir = ['Malcontent-' + libmalcontent_api_version,
+                       'Malcontent-' + libmalcontent_api_version + '.typelib']
+endif
 if get_option('ui').enabled()
   subdir('libmalcontent-ui')
 endif

meson_options.txt

@@ -15,3 +15,9 @@ option(
   value: 'enabled',
   description: 'enable UI library'
 )
+option(
+  'use_system_libmalcontent',
+  type: 'boolean',
+  value: false,
+  description: 'use installed libmalcontent rather than building it; used in distros to break a dependency cycle'
+)