meson/docs/markdown/snippets/fallback_bool.md

Controlling subproject dependencies with dependency(allow_fallback: ...)

As an alternative to the fallback keyword argument to dependency, you may use allow_fallback, which accepts a boolean value. If true and the dependency is not found on the system, Meson will fallback to a subproject that provides this dependency, even if the dependency is optional. If false, Meson will not fallback even if a subproject provides this dependency.