GNU/Hurd: Add getExecutablePath support

Author: sthibaul

cabal-install/src/Distribution/Client/Compat/ExecutablePath.hs

@@ -19,7 +19,7 @@ import Foreign.Marshal.Alloc
 import Foreign.Ptr
 import Foreign.Storable
 import System.Posix.Internals
-#elif defined(linux_HOST_OS)
+#elif defined(linux_HOST_OS) || defined(gnu_HOST_OS)
 import Foreign.C
 import Foreign.Marshal.Array
 import System.Posix.Internals
@@ -96,7 +96,7 @@ getExecutablePath = _NSGetExecutablePath >>= realpath
 --------------------------------------------------------------------------------
 -- Linux
 
-#elif defined(linux_HOST_OS)
+#elif defined(linux_HOST_OS) || defined(gnu_HOST_OS)
 
 foreign import ccall unsafe "readlink"
     c_readlink :: CString -> CString -> CSize -> IO CInt