Motivation for this change

This package uses CMake's install(EXPORT ...) command which assumes that libraries can be found relative to the CMake files. This results in the following error when this package is imported by another CMake package:

CMake Error at /nix/store/n77s62wwd9m7cjqd4hky91h2khkiy06n-libyaml-cpp-0.6.3-dev/lib/cmake/yaml-cpp/yaml-cpp-targets.cmake:74 (message):
  The imported target "yaml-cpp" references the file

     "/nix/store/n77s62wwd9m7cjqd4hky91h2khkiy06n-libyaml-cpp-0.6.3-dev/lib/libyaml-cpp.so.0.6.3"

  but this file does not exist.  Possible reasons include:

  * The file was deleted, renamed, or moved to another location.

  * An install or uninstall procedure did not complete successfully.

  * The installation package was faulty and contained

     "/nix/store/n77s62wwd9m7cjqd4hky91h2khkiy06n-libyaml-cpp-0.6.3-dev/lib/cmake/yaml-cpp/yaml-cpp-targets.cmake"

  but not all the files it references.

Call Stack (most recent call first):
  /nix/store/n77s62wwd9m7cjqd4hky91h2khkiy06n-libyaml-cpp-0.6.3-dev/lib/cmake/yaml-cpp/yaml-cpp-config.cmake:11 (include)
  src/rviz/CMakeLists.txt:1 (find_package)

Disabling multiple outputs only results in a small increase in closure size (936K -> 1.1M).

Things done

• Tested using sandboxing (nix.useSandbox on NixOS, or option sandbox in nix.conf on non-NixOS linux)
• Built on platform(s)
  • NixOS
  • macOS
  • other Linux distributions
• Tested via one or more NixOS test(s) if existing and applicable for the change (look inside nixos/tests)
• Tested compilation of all pkgs that depend on this change using nix-shell -p nixpkgs-review --run "nixpkgs-review wip"
• Tested execution of all binary files (usually in ./result/bin/)
• Determined the impact on package closure size (by running nix path-info -S before and after)
• Ensured that relevant documentation is up to date
• Fits CONTRIBUTING.md.

cc @andir