WebPConfig.cmake is not provided by this package, which seems to cause dependent projects to fail to find libwebp unless they include their own logic
Probably those projects should include their own logic anyways, then (it is as simple as using both pkg_check_modules and find_package and using the first one that succeeds), as it's hardly guaranteed that any given distributor will happen to use cmake. In fact, probably as a general rule of thumb projects depending on libwebp should use pkg_check_modules on linux and find_package on Windows.