calibrateMtimeChangeDelay takes 25s to execute on Mac OS X with HFS+ system

```
Prelude Distribution.Compat.Time Data.Time>
do { t1 <- getCurrentTime;
r <- calibrateMtimeChangeDelay;
t2 <- getCurrentTime;
print (t2 `diffUTCTime` t1);
return r; }
24.609871s
(1006168,1000000)
```

HFS+ stores file modtime with 1 second resolution. Function is here: https://github.com/haskell/cabal/blob/e14da30b01ff65098cc7fb20c134323c863a85ff/Cabal/Distribution/Compat/Time.hs#L184

Start in Cabal/Distribution/Compat/Time.hs at calibrateMtimeChangeDelay and reproduce the reported timing on Mac OS X with HFS+. Read the surrounding implementation to understand how the one-second HFS+ modification-time resolution is handled. Done means the calibration no longer takes roughly 25 seconds while retaining correct timing behavior.

Written by the indexing model from the issue text.