Change moreRecentFile test to workaround races

This changes `moreRecentFile a b` to return true not only when `a`
is younger than `b`, but also when `a` is exactly the same age of `b`, as
that case is subject to race-conditions, and it's better to err on assuming
it needs to be regenerated.

This is an attempt to provide a pragmatic workaround for #2311

Cabal/Distribution/Simple/Utils.hs

@@ -775,7 +775,7 @@ moreRecentFile a b = do
  then return True
  else do tb <- getModificationTime b
  ta <- getModificationTime a
- return (ta > tb)
+ return (ta >= tb)
 
 -- | Like 'moreRecentFile', but also checks that the first file exists.
 existsAndIsMoreRecentThan :: FilePath -> FilePath -> IO Bool