cmake: allow building liblphobos with system zlib

[liushuyu]

This pull request allows third-party distributors to build Phobos runtime shipped with LDC using the system zlib library.

Many Linux distributions ship their own patches to do this; this pull request tries to avoid that and provides a unified way to achieve this goal.

[kinke]

Thanks. What's missing is making the compiler link the library, especially/only(?) when linking static Phobos. E.g., by adding it as defaultlib in ldc2.conf - something like -defaultlib=phobos2-ldc,z,druntime-ldc.

[kinke]

Superseded by #4742.