[cmake] Check the availability of `bzip2.pc'. (CMakeLists.txt): Check the availability of `bzip2.pc'. * If `bzip2.pc' is available, Requires.private should include bzip2, but Libs.private should not include -lbz2. * If `bzip2.pc' is unavailable, Requires.private cannot include bzip2, but Libs.private should include -lbz2. Fix #897.