My project (in C) has a third party dependency at build time. But the third party library is, by default, installed to `/opt/` instead of `/lib`, and I cannot find it in `pkg-config`. From `mesonbuild`'s documentation, should I use `declare_dependency`, I don't have its source code to treat it as my sub-project. If I use `dependency()` to define it, I can't find the correct argument to define a customized location.

How to declare dependency for a non-standard third party library?