Allow reusing system libraries

[xaverdh]

Currently the build system vendors opencv, dlib, googletest. It would be nice to allow reusing system libraries for this.
The build system could use an overhaul in general.

[dpar39]

Hi @xaverdh, right now I'm in the process of porting all development from a private repo to this public project since I'm still seeing interest in this project. A lot has changed since I forked it (e.g. now the build system is Bazel because I'm using MediaPipe instead of dlib for face landmark detection, among many things). I should finish putting everything in place sometime this upcoming week, stay tuned.