[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.
1 file changed