Why does the lib_source need to be labels?

Very often the external lib repo does not have a :srcs target to be used here:
https://github.com/bazelbuild/rules_foreign_cc/blob/0258d350a9ff4c5145f131b7247d161a432dec8d/examples/make_simple/BUILD.bazel#L17

For example, https://github.com/pciutils/pciutils has no BUILD setup. If I can add a BUILD setup there, I wouldn't need to use these external targets, right?

So, how can I externally build that repo lib, and use it as a dependency label in my bazel flow? It's only a make call, but I need this tool to sort out the cached repo path.

Start with the linked examples/make_simple/BUILD.bazel line and inspect how lib_source is expected to be supplied. Compare that flow with the pciutils repository, which has no BUILD setup, and review the existing external Make integration. Done means establishing a defined approach for building such a repository externally and consuming its result as a dependency label.

Written by the indexing model from the issue text.