compositor-x11: Remove support for ancient XCB

compositor-x11: Remove support for ancient XCB

We had two non-pkg-config check paths in the configure script, to
support XCB functionality used before XCB had had an accompanying
release: xcb_poll_for_queued_event (released in 1.8, 2012), and a
usable XKB event mechanism (released in 1.9, 2013).

Convert the former to a version-based hard dependency, and the latter to
a version-based soft dependency. This avoids two compiler checks.

Signed-off-by: Daniel Stone <daniels@collabora.com>
Reviewed-by: Quentin Glidic <sardemff7+git@sardemff7.net>