configure: check for core xcb and link libGL against it

configure: check for core xcb and link libGL against it

Make sure to check the presence of the module in order to pick the
correct libs flag and before feeding them to the compiler/linker.

Current libGL depends conditionally (when building with dri3) upon
xcb 1.9.3 and unconditionally on ancient xcb functions -
xcb_generate_id and xcb_request_check amongst others.

v2: Use PKG_CHECK_EXISTS() when checking for dri3 xcb.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=80848
Signed-off-by: Emil Velikov <emil.l.velikov@gmail.com>