cmake : fix usage issues (#1257)

cmake : fix usage issues (#1257)

* CMake config: Create target only once

Fix error on repeated find_package(ggml).
For simplicity, check only for the top-level ggml::ggml.

* CMake config: Add CUDA link libs

* CMake config: Add OpenCL link libs

* CMake config: Use canonical find_dependency

Use set and append to control link lib variables.
Apply more $<LINK_ONLY...>.

* CMake config: Wire OpenMP dependency