[Feature] support external_dependencies via pkg-config

[mrcjkb]

luarocks, as usual, uses its hand-rolled heuristics to detect external dependencies.
Perhaps we can use something like pkg-config, for which there exists a rust wrapper library.

• We may also want to provide a way to let rocks know about external dependencies for environments that don't come with pkg-config.
• It might also be a good idea to add a flag for not failing if external dependencies are not found (as they are not needed for installation), but to print a warning message instead (should this be the default behaviour?)