Make path to pre-built version of librdkafka more flexible

[cstyles]

Sister PR to https://github.com/fede1024/rust-rdkafka/pull/787.

The build script currently expects a very specific filesystem layout in order to statically link. If you check out the source code for librdkafka somewhere and compile it with ./configure and make, the build artifacts will exist in the src/ directory under the project directory and everything will work. The rdkafka-sys crate does something similar behind the scenes by default.

However, sometimes we already have librdkafka built for us. For example, when installed via a package manager like Homebrew or APT, we will have the build artifacts available somewhere like /opt/homebrew/lib or /usr/lib/aarch64-linux-gnu. In those cases, the automatic appending of src/ to the path in DEP_LIBRDKAFKA_STATIC_ROOT is problematic.

Taking a page from jemallocator^1, another Rust crate that wraps a C/C++ library, we can instead accept a path to the static library file and pass its parent (the directory in which it resides) to cargo:rustc-link-search. This supports both the case where we have the library installed via a package manager (by setting DEP_LIBRDKAFKA_STATIC_ROOT to something like /opt/homebrew/lib/librdkafka.a) and the case where we're building from source ourselves (by pointing to something like ~/Downloads/librdkafka/src/librdkafka.a).