Use OpenSSL and ZLIB targets to link against

[lorteddie]

Both libraries are binary compatible between versions so there is no need to depend on specific build paths of them. If available their targets should be preferred.

[VivienDelmon]

These patchs are interesting and allow to further deploy dcmtk without hard-coded path related to the build machine. You should just use find_dependency instead of find_package for better error messages in CMake/DCMTKConfig.cmake.in see https://stackoverflow.com/questions/50263280/difference-between-find-dependency-and-find-package-in-cmake