Add a flag for dynamic date range evaluation in patterns.
By default, Mutt evaluates relative date patterns, such as "~d<1d" against the date when the pattern is compiled. For index-format-hook, we need to pattern match against a message based on the current date/time, so that for example, conditional date format patterns correctly match after Mutt has been open for many days/weeks. Add a flag, MUTT_PATTERN_DYNAMIC, that changes ~d and ~r evaluation to reevaluate the min/max range with each match. This will of course be slower, but for the index, where a screenful of messages at a time are evaluated, is an acceptable tradeoff against accurate pattern matching.
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