* imgui: no packages for debian, author suggests using in the source tree
* miniz: No packages, author suggests using in the source tree
* qhull: libqhull-dev does not contain libqhullcpp => link errors. Until it is fixed, we will use the builtin version. https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=925540
* semver: One module C library, author expects to use clib for installation. No packages.
* catch2: Only Arch has packages for catch2, other distros at most catch (v1.x). Being strictly header only, we bundle this in the source tree. Used for the unit-test suites.