FindPkgConfig: set policies CMP0054 and CMP0057 to new
One may encounter warnings if FindPkgConfig is used in any project, even indirectly, that has set any of these policies to old explicitely or requires an older version.
One example where I found this is when building upstream curl using CMake, where this is hit through FindOpenSSL.
Backport: release
Topic-rename: FindPkgConfig-scope