Re haskell#7714: don't test nroff-ability of cabal man --raw under Windows

Windows does not have `nroff` (neither has it `man`) and `cabal man`
fails there usually for lack of these backends.

Author: andreasabel

cabal-testsuite/PackageTests/Manpage/cabal.test.hs

@@ -1,21 +1,24 @@
 import System.Process
+import Distribution.System (OS(Windows,Linux,OSX), buildOS)
 import Test.Cabal.Prelude
 
-
 main = cabalTest $ do
     r <- cabal' "man" ["--raw"]
     assertOutputContains ".B cabal install" r
     assertOutputDoesNotContain ".B cabal manpage" r
 
-    -- Check that output of `cabal man --raw` can be passed through `nroff -man`
-    -- without producing any warnings (which are printed to stderr).
-    --
-    -- NB: runM is not suitable as it mixes stdout and stderr
-    -- r2 <- runM "nroff" ["-man", "/dev/stdin"] $ Just $ resultOutput r
-    (ec, _output, errors) <- liftIO $
-      readProcessWithExitCode "nroff" ["-man", "/dev/stdin"] $ resultOutput r
-    unless (null errors) $
-      assertFailure $ unlines
-        [ "Error: unexpected warnings produced by `nroff -man`:"
-        , errors
-        ]
+    -- The following test of `cabal man` needs `nroff` which is not available under Windows.
+    unless (buildOS == Windows) $ do
+
+      -- Check that output of `cabal man --raw` can be passed through `nroff -man`
+      -- without producing any warnings (which are printed to stderr).
+      --
+      -- NB: runM is not suitable as it mixes stdout and stderr
+      -- r2 <- runM "nroff" ["-man", "/dev/stdin"] $ Just $ resultOutput r
+      (ec, _output, errors) <- liftIO $
+        readProcessWithExitCode "nroff" ["-man", "/dev/stdin"] $ resultOutput r
+      unless (null errors) $
+        assertFailure $ unlines
+          [ "Error: unexpected warnings produced by `nroff -man`:"
+          , errors
+          ]