propagate 'use re eval' into return from (??{})

propagate 'use re eval' into  return from (??{})

(??{}) returns a string which needs to be put through the regex compiler,
and which may also contain (?{...}) - so any 'use re eval' in scope needs
to be propagated into the inner environment. Achieve this by adding a new
private flag - PREGf_USE_RE_EVAL - to the regex to indicate the use is in
scope, and modify how the call to compile the inner pattern is done,
to allow the use state to be passed in.