Export CMake targets to the build tree

Properly export Vorbis targets when added via `add_subdirectory()`.
2 jobs for export-cmake-targets-to-the-build-tree in 1 minute and 43 seconds (queued for 1 second)
Status Job ID Name Coverage
  Build
passed autoconf #10891
docker

00:01:43

passed cmake #10892
docker

00:00:27