Build system should not assume everything is vendored and bundled

I think this project should update build system so that it does not assume everything is vendored and bundled. It should support normal methods (such as pkg-config) to detect the required dependencies when they are provided by the system and build using them and install using normal methods.

Preliminary work has been done in #191 but more work is needed.

This is a feature request not a bug report.

Rationale

This feature would make it much easier for either the maintainers of the project, or downstream distributions, to package and distribute this project in a variety of binary formats.

To my knowledge this feature was first suggested a year ago in a comment https://github.com/Murmele/Gittyup/issues/198#issuecomment-1161543223 but I think the feature should be discussed by itself regardless of which binary formats the project chooses to distribute as no one binary format will be universally acceptable to everyone, including #579.

Start by reviewing the preliminary work in issue #191 and the related distribution discussion in #579. Identify how dependencies are currently vendored and bundled, then define the build and installation changes needed to use system-provided dependencies through normal detection methods such as pkg-config. Done means system dependencies can be detected and used without requiring everything to be bundled.