Add prefix to pkgconfig root

[bruchar1]

Changelog: Bugfix: Add prefix var and any custom content (through the pkg_config_custom_content property) to already generated pkg-config root .pc files by PkgConfigDeps.
Docs: omit

Fixes #14049

[memsharded]

Looks good, thanks!
Assigning @franramirez688 for final review

[franramirez688]

Thanks for the contribution @bruchar1

I'm wondering why this is needed. Which is the use case? Was it actually a bug?

[bruchar1]

I'm wondering why this is needed. Which is the use case? Was it actually a bug?

This is detailed in the linked issue.

The fact the prefix variable is not in the .pc file is not really a bug, but having it solves issues with meson integration, especially because meson needs this variable to know where the package is installed.

The fact pkg_custom_content does not work with the root .pc file from a package with components could be considered as a bug.

[franramirez688]

This is detailed in the linked issue.

Oh, so sorry, my fault, I did not realize about the issue. Let me have a look at it. Perhaps, it makes sense to add a dedicated test for this change.

[franramirez688]

Added this extra check to ensure that the custom content is being added only to the root .pc file.