Guard quickcheck.1 regeneration

Without this guard it will be regenerate on each run,
and cabal new-build considers whole package dirty.

Setup.hs

@@ -14,6 +14,7 @@ import Distribution.Simple (
 import Distribution.Simple.Setup ( SDistFlags )
 
 import System.Process ( system )
+import System.Directory ( doesFileExist, getModificationTime )
 
 #ifndef MIN_VERSION_cabal_doctest
 #define MIN_VERSION_cabal_doctest(x,y,z) 0
@@ -61,10 +62,20 @@ main = defaultMainWithHooks myHooks
 --
 myPreSDist :: Args -> SDistFlags -> IO HookedBuildInfo
 myPreSDist _ _ = do
-  putStrLn "Building the man page (shellcheck.1) with pandoc..."
-  putStrLn pandoc_cmd
-  result <- system pandoc_cmd
-  putStrLn $ "pandoc exited with " ++ show result
+  exists <- doesFileExist "shellcheck.1"
+  if exists
+  then do
+    source <- getModificationTime "shellcheck.1.md"
+    target <- getModificationTime "shellcheck.1"
+    if target < source
+    then makeManPage
+    else putStrLn "shellcheck.1 is more recent than shellcheck.1.md"
+  else makeManPage
   return emptyHookedBuildInfo
   where
+    makeManPage = do
+      putStrLn "Building the man page (shellcheck.1) with pandoc..."
+      putStrLn pandoc_cmd
+      result <- system pandoc_cmd
+      putStrLn $ "pandoc exited with " ++ show result
     pandoc_cmd = "pandoc -s -t man shellcheck.1.md -o shellcheck.1"

ShellCheck.cabal

@@ -32,6 +32,7 @@ Extra-Source-Files:
 custom-setup
   setup-depends:
     base          >= 4     && <5,
+    directory     >= 1.2   && <1.4,
     process       >= 1.0   && <1.7,
     cabal-doctest >= 1.0.6 && <1.1,
     Cabal         >= 1.10  && <2.5