In case of static link, SFML tries to locate the libraries it depends on, in standard paths and in ${SFML_ROOT}/lib (i.e. the same search paths as for the SFML libraries themselves).
If they are found, they are added to ${SFML_DEPENDENCIES} and ${SFML_XXX_DEPENDENCIES}, where XXX is the name of the corresponding module.
If one or more are not found, the expected behaviour is triggered (fatal error if REQUIRED, nothing if QUIET, information message otherwise).
I haven't really thought about customization yet, so suggestions are welcome