gclue-simple: Add portal support

When running inside a flatpak sandbox, or when GTK_USE_PORTAL
is set in the environment, talk to the location portal instead
of GeoClue directly.

In this situation, gclue_simple_get_client will return NULL.
3 jobs for portal-support in 6 minutes and 36 seconds (queued for 1 second)
latest
Status Job ID Name Coverage
  Test
failed #40775
ubuntu-16.04

00:06:36

passed #40776
ubuntu-17.10

00:06:15

passed #40777
ubuntu-18.04

00:06:10

 
Name Stage Failure
failed
ubuntu-16.04 Test
                                              ^
../libgeoclue/gclue-simple.c:436:9: note: in expansion of macro 'g_set_object'
g_set_object (&priv->location, location);
^
ninja: build stopped: subcommand failed.
Uploading artifacts...
/builds/matthiasc/geoclue/build: found 118 matching files
Uploading artifacts to coordinator... ok
id=40775 responseStatus=201 Created token=nKRSdz-w
ERROR: Job failed: exit code 1