- Add options to the autotools build to set the path for the "vendor"
or "extra" configuration snippets, functions and completions
directories.
- Remove the vendor_completions directory from the Xcode build, as
these are relocatable and compiling the paths in does not make sense.
This allows packaging tools like Homebrew and Nix to use a common
directory outside of the main prefix for third-party completions, and
to make these available for programmatic discovery through `pkg-config`.
Closes#2113
