Resolve "Sight does not build because of libs/__/service/extension/config.hpp"

Description

• The generate_headers_export.cmake is now well called only at install time, at the beginning of each target.
• Only do the specific process for config.hpp on top-level files of targets (this will exclude /**/config.hpp)

Closes #1174

How to test it?

• Check that an installed application works
• Build with an external repository