fix windows assistant upgrade glitch

Prevent windows assistant from trying (and failing) to upgrade itself,
which has never been supported on windows.

The new windows build is made with UPGRADE_LOCATION set, which enabled this
code path that had never run on windows before, and doesn't work. I don't
want to try to support self-upgrade on windows, or generally on other OS's
than the ones where its working, so added a check for that. This way the
build can keep setting UPGRADE_LOCATION and if some later git-annex does
learn how to upgrade itself on some OS, it won't need changing the build
setup.

Assistant/Threads/Upgrader.hs

@@ -31,7 +31,7 @@ import qualified Data.Text as T
 
 upgraderThread :: UrlRenderer -> NamedThread
 upgraderThread urlrenderer = namedThread "Upgrader" $
-	when (isJust BuildInfo.upgradelocation) $ do
+	when upgradeSupported $ do
 		{- Check for upgrade on startup, unless it was just
 		 - upgraded. -}
 		unlessM (liftIO checkSuccessfulUpgrade) $

Assistant/Upgrade.hs

@@ -338,6 +338,18 @@ distributionInfoUrl = fromJust BuildInfo.upgradelocation ++ ".info"
 distributionInfoSigUrl :: String
 distributionInfoSigUrl = distributionInfoUrl ++ ".sig"
 
+{- Upgrade only supported on linux and OSX. -}
+upgradeSupported :: Bool
+#ifdef linux_HOST_OS
+upgradeSupported = isJust BuildInfo.upgradelocation
+#else
+#ifdef darwin_HOST_OS
+upgradeSupported = isJust BuildInfo.upgradelocation
+#else
+upgradeSupported = False
+#endif
+#endif
+
 {- Verifies that a file from the git-annex distribution has a valid
  - signature. Pass the detached .sig file; the file to be verified should
  - be located next to it.

Assistant/WebApp/Configurators/Preferences.hs

@@ -21,7 +21,7 @@ import Annex.NumCopies
 import Utility.DataUnits
 import Git.Config
 import Types.Distribution
-import qualified BuildInfo
+import Assistant.Upgrade
 
 import qualified Data.Text as T
 import qualified System.FilePath.ByteString as P
@@ -59,7 +59,7 @@ prefsAForm d = PrefsForm
 		, ("disabled", NoAutoUpgrade)
 		]
 	autoUpgradeLabel
-		| isJust BuildInfo.upgradelocation = "Auto upgrade"
+		| upgradeSupported = "Auto upgrade"
 		| otherwise = "Auto restart on upgrade"
 
 	positiveIntField = check isPositive intField