Switch from the old AM_PATH_CHECK macro to pkg-config.

check 0.9.4 is now required to build XCB's unit tests.

The version that we were requiring was not actually new enough to let
our unit tests compile, and the AM_PATH_CHECK macro is now considered
deprecated. We know that versions of check using pkg-config are new
enough to work, and the check dependency was optional anyway, so we've
dropped support for older versions.