configure: add xcb cflags and extralibs to cflags and extralibs.

Replace xcb_event by xcb, as the former is no more used and the latter
is needed on configurations where only xcb is available, and not
libxcb_shm and/or libxcb_xfixes.

Signed-off-by: Benoit Fouet <benoit.fouet@free.fr>
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>