Make inotify optional.

Former-commit-id: 7b1776384f5755edf4d792a5c3c73ca4f5ef66e2

posix/Reanimate/FileWatch.hs

@@ -0,0 +1,18 @@
+module Reanimate.FileWatch
+  ( watchFile ) where
+
+import Control.Concurrent
+import           System.Directory
+
+watchFile :: FilePath -> IO () -> IO ()
+watchFile path handler = do
+    t0 <- getModificationTime path
+    forkIO $ loop t0
+    return ()
+  where
+    loop lastModTime = do
+      t1 <- getModificationTime path
+      if t1 /= lastModTime
+        then handler >> loop t1
+        else threadDelay (10^3 * delay) >> loop lastModTime
+    delay = 1000 -- pool delay in ms.