Currently, building vala bindings from a tarball is broken because
spice-client-glib-2.0.deps is missing from the tarball. This commit
adds it to EXTRA_DIST and also makes sure the vala bindings don't
get built/installed unless --enable-vala has been passed to configure.
This means vala must be installed to build the vala bindings from a
tarball. Fixes fdo bug #44000.