FindPkgConfig: Write variables to cache for newer versions of pkg-config

This affects the FindPkgConfig module.

There is a bug when newer versions of pkg-config are installed which causes the variables ${_prefix}_INCLUDE_DIRS and ${_prefix}_CFLAGS_OTHER to only get set in the current scope and not written to the cache. Subsequent runs of CMake would cause the variables to not be set and the build to fail.

For instance running Ubuntu 22.04 (version 0.29.2 of pkg-config) works as expected but running Ubuntu 23.04 (version 1.8.1) breaks.

Issue: #25377 (closed)
Topic-rename: FindPkgConfig-caching