Tweak variable names in core/zip/CMakeLists.txt.
Tweak variable names in core/zip/CMakeLists.txt.

We have three types of headers in this module:

- Those related to ROOT's functionality.
- Those related to ROOT's internal implementation of zlib.
- Those related to ROOT's old custom compression algorithm.

This commit attempts to get sane naming scheme going in the CMake
variable names.

karies/root,root-mirror/root,root-mirror/root,olifre/root,karies/root,olifre/root,karies/root,karies/root,olifre/root,root-mirror/root,olifre/root,karies/root,root-mirror/root,olifre/root,karies/root,karies/root,root-mirror/root,karies/root,root-mirror/root,olifre/root,karies/root,olifre/root,olifre/root,root-mirror/root,karies/root,olifre/root,karies/root,root-mirror/root,olifre/root,olifre/root,root-mirror/root,root-mirror/root,root-mirror/root