Added --disable-snappy and --disable-lz4 to build-snapshot

The build-snapshot script fetches Git sources and packages them into a tarball
which it then feeds to build-complete. In doing so it runs configure with
staticly defined options, causing this phase to implicitly depend on both snappy
and lz4 development libraries. This change removes that dependency.

Signed-off-by: Samuli Seppänen <samuli@openvpn.net>