A few things cause the cmake build install a libz.so that is not bit-for-bit identical with the one produced by the make build. A couple of these might be worth fixing.

• no default buildtype, so if you forget to set Release, you get a funny build
• -DHAVE_HIDDEN missing from defines on linux
• object files linked in different order
• Makefile links against -lc explicitly

Found while investigating #485