configure: fix default location for fish completions

Fish defines a vendor completions directory for completions that are not
installed as part of the fish project itself, and the vendor completions
are preferred if they exist. This prevents trying to overwrite the
builtin curl.fish completion (or creating file conflicts in distro
packaging).

Prefer the pkg-config defined location exported by fish, if it can be
found, and fall back to the correct directory defined by most systems.

Closes #3723
Reviewed-by: Daniel Gustafsson