Enable targets exportation during build

[wgoadri]

close #149

[wgoadri]

@nmellado @azaleostu I modified the cmake exportation to make the targets available during the build. Now, projects that are exported can link to Ponca. The problem only appeared with libraries.

[wgoadri]

I also did a bit of work on the installation folder to add the Ponca name. If Ponca is installed by another project, this makes the install directory more structured. It should not interfere with the previous installation because of the cmake find_package procedure.

Also, I think that the installation paths could be shared by the cmake files, they are currently hard coded.

[azaleostu]

Maybe update the changelog as this might break existing installs, otherwise looks good 👍

[nmellado]

I need to understand why the tests didn't start.

[wgoadri]

I think you need to approve the PR to trigger the action because I am not a contributor.
To avoid this, the workflow can use the pull_request_target event that allows fork from anyone to trigger the test with a pr from a fork. But it is not secure.

https://docs.github.com/en/repositories/managing-your-repositorys-settings-and-features/enabling-features-for-your-repository/managing-github-actions-settings-for-a-repository#controlling-changes-from-forks-to-workflows-in-public-repositories
actions/add-to-project#163