meson/docs/markdown/snippets/implicit_fallback.md

Implicit dependency fallback

dependency('foo') now automatically fallback if the dependency is not found on the system but a subproject wrap file or directory exists with the same name.

That means that simply adding subprojects/foo.wrap is enough to add fallback to any dependency('foo') call. It is however requires that the subproject call meson.override_dependency('foo', foo_dep) to specify which dependency object should be used for foo.