Improve CMake reuse of vendored subprojects

[Growl1234]

• Fix CMake target reuse so that subprojects already introduced by nested FetchContent/subproject builds are not added again, avoiding duplicate targets such as test-drive, test-drive-lib, and related test executables.
• Add Meson dist symlinks, so the CMake build can reuse the dependency sources already included in the dist tarball instead of downloading another copy online.