CMake: Configured `zconf.h` may be incorrect depending on SCM software

https://github.com/madler/zlib/commit/3d6dc3c3aa73fc5421a7671b32d1dd251fea1edc broke configuration of `zconf.h` due to hard-coded offsets for the inserted text when using SCM software that expands special placeholders in files since that file contains `@(#) $Id$`, which might get replaced with some text causing the offset to be incorrect.