Cmake export fix

[felix2010]

I tried to fix the generation of the file yaml-cpp-config.cmake. Former the variables
YAML_CPP_CMAKE_DIR and YAML_CPP_LIBRARIES where empty within this file,
now they are set appropriately.

I reworked the file accordingly to cmake documentation as described in the commit message.

[jbeder]

What behavior will this fix? (That is, how do you reproduce the problem you're seeing, to verify that this fixes it?)

Once you specify that (e.g. sequence of command lines), let's put them in the automated github tests so we can keep these passing.

[felix2010]

Configure the project and check the generated yaml-cpp-config.cmake

cmake -B build && grep -E '\(YAML_CPP_(LIBRARIES|INCLUDE)' build/yaml-cpp-config.cmake

Actual output is

set(YAML_CPP_INCLUDE_DIR "")
set(YAML_CPP_LIBRARIES "")

but expected are non empty variables, i.e.

set_and_check(YAML_CPP_INCLUDE_DIR "${PACKAGE_PREFIX_DIR}/include")
set(YAML_CPP_LIBRARIES "yaml-cpp")

I'm not familiar with github workflow/test, thus I need your advice how to bring this into the automated tests.

[jbeder]

Thanks! And what's the goal of having those variables set? That is, what subsequent command with use them and expect them to be nonempty?

[felix2010]

The variables are documented entities in yaml-cpp-config.cmake introduced with a397ad2.
Afaik these are convenience variables that each cmake package should provide. Initially the variables got expanded in the right way, but this behavior was brocken in 5e9cb01.

ubuntu/jammy ships with a yaml-cpp-config.cmake that sets the variables in a correct way, but only because the maintainer of the package patched the sources (content of yaml-cpp_0.6.2-4ubuntu1.debian.tar.xz from https://packages.ubuntu.com/source/focal/yaml-cpp).

yaml-cpp is a great library, I like it and I like to have it in my preferred linux distribution. If we require patches from all distribution integrators we risk, that distributions ship with old versions of the library or dropping it from the distro.
That was the main motivation for my PR.

[jbeder]

I'm going to merge this because I'm too lazy to try to figure out how to test it properly, and it very likely is the right solution. At some point, it would be nice to have a way to know if this is broken, so I'm open to PRs that just have a new test case in the Github actions file.

[jbeder]

Fixes #774