Optional dependencies are promoted to required dependencies if the specified
library is enabled via --enable-optional-components or enabled_component = 1,
or dropped if the specified library is disabled. Of course, another mechanism
(such as preprocessor macros) needs to be used so that there are no unresolved
It's a warning to specify a library that's not optional in optional_libraries.
Such a dependency will be treated as if it were provided via required_libraries.