build: Reintroduce enable-gvfs-metadata option

Gedit used to have an enable-gvfs-metadata to enable using gvfs to store
metadata. This option was removed with the port to meson. Reintroduce it and set
the default to 'auto' (defaults to 'yes' when running in linux, to 'no'
otherwise).

Closes: #141
1 job for master in 6 minutes and 26 seconds (queued for 2 seconds)
Status Job ID Name Coverage
  Test
passed #270727
flatpak

00:06:26