Make compatible again for GHC back to 7.4.1

By avoiding `import Foreign` this accomplishes backward-compatibility
w.r.t. the `unsafePerformIO` relocation w/o resorting to CPP
conditionals.

Moreover, in order to support base<4.7 this commits needs to reintroduce
a harmless backport of `newFilePath`.

Signed-off-by: Herbert Valerio Riedel <hvr@gnu.org>

System/Posix/Env.hsc

@@ -39,6 +39,12 @@ import Control.Monad
 import Data.Maybe (fromMaybe)
 import System.Posix.Internals
 
+#if !MIN_VERSION_base(4,7,0)
+-- needed for backported local 'newFilePath' binding in 'putEnv'
+import GHC.IO.Encoding (getFileSystemEncoding)
+import qualified GHC.Foreign as GHC (newCString)
+#endif
+
 -- |'getEnv' looks up a variable in the environment.
 
 getEnv :: String -> IO (Maybe String)
@@ -128,6 +134,11 @@ putEnv keyvalue = do s <- newFilePath keyvalue
                      -- According to SUSv2, the string passed to putenv
                      -- becomes part of the enviroment. #7342
                      throwErrnoIfMinus1_ "putenv" (c_putenv s)
+#if !MIN_VERSION_base(4,7,0)
+    where
+      newFilePath :: FilePath -> IO CString
+      newFilePath fp = getFileSystemEncoding >>= \enc -> GHC.newCString enc fp
+#endif
 
 foreign import ccall unsafe "putenv"
    c_putenv :: CString -> IO CInt

System/Posix/Files/Common.hsc

@@ -85,11 +85,15 @@ import System.Posix.Error
 import System.Posix.Types
 import System.IO.Unsafe
 import Data.Bits
+import Data.Int
 import Data.Time.Clock.POSIX
 import Data.Ratio
 import System.Posix.Internals
-import Foreign
 import Foreign.C
+import Foreign.ForeignPtr
+import Foreign.Marshal
+import Foreign.Ptr
+import Foreign.Storable
 
 -- -----------------------------------------------------------------------------
 -- POSIX file modes

System/Posix/Process/Common.hsc

@@ -313,11 +313,6 @@ getProcessStatus block stopped pid =
       _  -> do ps <- readWaitStatus wstatp
 	       return (Just ps)
 
-#if __GLASGOW_HASKELL__ < 701
--- not available prior to 7.1
-#define interruptible safe
-#endif
-
 -- safe/interruptible, because this call might block
 foreign import ccall interruptible "waitpid"
   c_waitpid :: CPid -> Ptr CInt -> CInt -> IO CPid

System/Posix/Signals.hsc

@@ -103,8 +103,12 @@ module System.Posix.Signals (
   -- siginterrupt
   ) where
 
-import Foreign
+import Data.Word
 import Foreign.C
+import Foreign.ForeignPtr
+import Foreign.Marshal
+import Foreign.Ptr
+import Foreign.Storable
 import System.IO.Unsafe (unsafePerformIO)
 import System.Posix.Types
 import System.Posix.Internals