`NIX_LDFLAGS` typically contains just `-rpath` and `-L`, which we already
handle. However, at least one setup hook in Nixpkgs [0] adds a linkage
directive to it. To prevent library paths from being missed (as I've
observed myself with `NIX_LDFLAGS` being `-liconv ...`, making it so that
*all* paths are missed), let's just skip over them.

[0]: https://github.com/NixOS/nixpkgs/blob/08f615eb1b3c5cf7481996f29c6092c7c891b15b/pkgs/development/libraries/libiconv/setup-hook.sh