Do not find XCB's XINPUT library by default.
Review Request #125935 - Created Nov. 3, 2015 and submitted
The XINPUT library is unlikely to be available on many systems, including a lot of Linux distributions. This will warn developers in advance by (a) not searching for XINPUT unless it was explicitly requested, and (b) printing a warning if they do request it.
wacomtablet's kf5-port branch still configures, but with a warning. kwin configures without a warning. Modified kwindowsystem to call find_package(XCB), and that also configures without a warning.