Add compatibility implementation of unsetEnv.

Signed-off-by: Edward Z. Yang <ezyang@cs.stanford.edu>

Cabal/Distribution/Compat/Environment.hs

@@ -3,15 +3,24 @@
 {-# OPTIONS_HADDOCK hide #-}
 
 module Distribution.Compat.Environment
-       ( getEnvironment, lookupEnv, setEnv )
+       ( getEnvironment, lookupEnv, setEnv, unsetEnv )
        where
 
 import Prelude ()
 import Distribution.Compat.Prelude
 
+#ifndef mingw32_HOST_OS
+#if __GLASGOW_HASKELL__ < 708
+import Foreign.C.Error (throwErrnoIf_)
+#endif
+#endif
+
 import qualified System.Environment as System
 #if __GLASGOW_HASKELL__ >= 706
 import System.Environment (lookupEnv)
+#if __GLASGOW_HASKELL__ >= 708
+import System.Environment (unsetEnv)
+#endif
 #else
 import Distribution.Compat.Exception (catchIO)
 #endif
@@ -86,3 +95,34 @@ setEnv_ key value = do
 foreign import ccall unsafe "setenv"
    c_setenv :: CString -> CString -> CInt -> IO CInt
 #endif /* mingw32_HOST_OS */
+
+#if __GLASGOW_HASKELL__ < 708
+
+-- | @unsetEnv name@ removes the specified environment variable from the
+-- environment of the current process.
+--
+-- Throws `Control.Exception.IOException` if @name@ is the empty string or
+-- contains an equals sign.
+--
+-- @since 4.7.0.0
+unsetEnv :: String -> IO ()
+#ifdef mingw32_HOST_OS
+unsetEnv key = withCWString key $ \k -> do
+  success <- c_SetEnvironmentVariable k nullPtr
+  unless success $ do
+    -- We consider unsetting an environment variable that does not exist not as
+    -- an error, hence we ignore eRROR_ENVVAR_NOT_FOUND.
+    err <- c_GetLastError
+    unless (err == eRROR_ENVVAR_NOT_FOUND) $ do
+      throwGetLastError "unsetEnv"
+#else
+unsetEnv key = withFilePath key (throwErrnoIf_ (/= 0) "unsetEnv" . c_unsetenv)
+#if __GLASGOW_HASKELL__ > 706
+foreign import ccall unsafe "__hsbase_unsetenv" c_unsetenv :: CString -> IO CInt
+#else
+-- HACK: We hope very hard that !UNSETENV_RETURNS_VOID
+foreign import ccall unsafe "unsetenv" c_unsetenv :: CString -> IO CInt
+#endif
+#endif
+
+#endif