Build fails if ninja isn't aware of the byproducts for libclarabel_c

This commit allows Clarabel.cpp to be consumed by other CMake projects
via a call to `add_subdirectory`:

```cmake

add_subdirectory(path/to/clarabel.cpp)
```

Other projects might have Clarabel.cpp as a git submodule, and this
makes it easier to do so.

Including iostream means introducing the static (global) constructors
and destructors for std::cin, std::cerr, and std::cout. That extra
init and fini code is undesirable when those streams are not actually
used.