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