support building with yesod-default 1.1.0

Old 1.0.1 version is still supported as well. Cabal autodetects
which version is available, but in the Makefile, WITH_OLD_YESOD
has to be configured appropriately.

I have not squashed all the $newline warnings with the new Yesod.
They should go away eventually anyway as Yesod moves past that transition.

git-annex.cabal

@@ -40,6 +40,12 @@ Flag Assistant
 Flag Webapp
   Description: Enable git-annex webapp
 
+Flag CurrentYesod
+  Description: Using a current version of Yesod
+
+Flag OldYesod
+  Description: Using an old version of Yesod
+
 Executable git-annex
   Main-Is: git-annex.hs
   Build-Depends: MissingH, hslogger, directory, filepath,
@@ -73,8 +79,15 @@ Executable git-annex
     Build-Depends: dbus
     CPP-Options: -DWITH_DBUS
 
-  if flag(Webapp)
-    Build-Depends: yesod, yesod-static, yesod-default, case-insensitive,
+  if flag(CurrentYesod)
+    Build-Depends: yesod-default (>= 1.1.0)
+
+  if flag(OldYesod)
+    Build-Depends: yesod-default (<= 1.0.1.1)
+    CPP-Options: -DWITH_OLD_YESOD
+
+  if flag(Webapp) && (flag(CurrentYesod) || flag(OldYesod))
+    Build-Depends: yesod, yesod-static, case-insensitive,
      http-types, transformers, wai, wai-logger, warp, blaze-builder,
      blaze-html, crypto-api, hamlet, clientsession,
      template-haskell