caveats: suggest setting `CMAKE_PREFIX_PATH` when appropriate

Inspired by Homebrew/homebrew-core#235513.

It doesn't quite fix that issue, because `libomp` does not ship CMake
config files, but this would probably still be useful for keg-only
formulae that do ship CMake config files.

While we're here, let's replace the `pkg-config` references with the
now-canonical name `pkgconf`.