meson: fix pangocairo being optional

Cannot use dependency() directly in the structure, because it will
execute regardless of the option. Instead, let's store the dependency
name in the structure and use the same logic as with simple_clients to
conditionally look for the dependencies.

As a bonus, this brings friendly error messages to demo-clients
dependencies.

subsurfaces' dependencies are also converted to maintain consistency
with simple_clients.

Signed-off-by: Pekka Paalanen <pq@iki.fi>