configure aborts when some, but not all, xcb dependencies are met

If the qpa/xcb configure test passes, and the qpa/xcb-syslibs test fails, then
configure will fail unconditionally with the message "The test for linking against libxcb and support libraries failed!".

If the qpa/xcb test fails, however, configure will succeed if CFG_XCB is "auto" and at least one platform plugin is built.

What I expected: the qpa/xcb-syslibs test should work the same way as qpa/xcb: a failure should only be fatal if CFG_XCB is "yes", or if no other platform plugin is built.