build: Add an --enable-p2p configure option for the peer to peer feature
build: Add an --enable-p2p configure option for the peer to peer feature

A series of following commits will introduce a peer to peer feature for
pulling apps and runtimes from LAN peers and USB sticks without needing
an internet connection. This requires experimental API in libostree
(which needs to have been configured with --enable-experimental-api), so
needs to be hidden behind a configure option in flatpak too. It’s called
--enable-p2p, and bumps our libostree dependency to 2017.8 with
experimental API required too.

Signed-off-by: Philip Withnall <29c2a2ac2a854b9cf047275aa9de346d096be2e0@endlessm.com>

flatpak/flatpak,handsome-feng/flatpak,handsome-feng/flatpak,flatpak/flatpak,flatpak/flatpak,handsome-feng/flatpak,matthiasclasen/flatpak,matthiasclasen/flatpak,flatpak/flatpak,handsome-feng/flatpak,matthiasclasen/flatpak,flatpak/flatpak,matthiasclasen/flatpak,handsome-feng/flatpak,matthiasclasen/flatpak