build: Add -Dui option to Meson

This allows the UI components (libmalcontent-ui and malcontent-control)
to be disabled in the build so that a dependency cycle with flatpak can
be avoided (by building malcontent twice, once with `-Dui=disabled` and
then again with `-Dui=enabled`).

The dependency graph is:
  malcontent-control → libmalcontent-ui → flatpak → libmalcontent
which becomes cyclic if libmalcontent-ui and libmalcontent can only be
built at the same time.

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

Fixes: #16

meson.build

@@ -127,9 +127,13 @@ test_env = [
 
 subdir('accounts-service')
 subdir('libmalcontent')
+if get_option('ui').enabled()
   subdir('libmalcontent-ui')
+endif
 subdir('malcontent-client')
+if get_option('ui').enabled()
   subdir('malcontent-control')
+endif
 subdir('pam')
 subdir('po')
 

meson_options.txt

@@ -9,3 +9,9 @@ option(
   type: 'string',
   description: 'directory for PAM modules'
 )
+option(
+  'ui',
+  type: 'feature',
+  value: 'enabled',
+  description: 'enable UI library'
+)