Require pkg-config 0.24 for creating pkgconfig() dependencies

Require pkg-config 0.24 for creating pkgconfig() dependencies
- The support for --print-requires|provides was only upstreamed in
  pkg-config 0.24, prior to that it relied on distros patching their
  pkg-config. Rpm relying on non-upstream stuff is not good...
  Now that it's upstream we can make a clean break and simply not
  even try to generate the dependencies with anything older.
- The upstreamed version behaves slightly differently from what distros
  were patching with, for rpm's purposes we also want
  --print-requires-private now.