PDF: Better detection of a Cygwin environment.

Should close #5451.

Unlike the earlier fix, this one doesn't spill out to stderr
when 'uname -o' fails.

src/Text/Pandoc/PDF.hs

@@ -42,6 +42,7 @@ import Text.Pandoc.Error (PandocError (PandocPDFProgramNotFoundError))
 import Text.Pandoc.MIME (getMimeType)
 import Text.Pandoc.Options (HTMLMathMethod (..), WriterOptions (..))
 import Text.Pandoc.Process (pipeProcess)
+import System.Process (readProcessWithExitCode)
 import Text.Pandoc.Shared (inDirectory, stringify)
 import qualified Text.Pandoc.UTF8 as UTF8
 import Text.Pandoc.Walk (walkM)
@@ -89,7 +90,13 @@ makePDF program pdfargs writer opts doc =
       -- user names (see #777)
       let withTempDir templ action = do
             tmp <- getTemporaryDirectory
-            if '~' `elem` tmp
+            uname <- E.catch
+              (do (ec, sout, _) <- readProcessWithExitCode "uname" ["-o"] ""
+                  if ec == ExitSuccess
+                     then return $ Just sout
+                     else return Nothing)
+              (\(_ :: E.SomeException) -> return Nothing)
+            if '~' `elem` tmp || uname == Just "Cygwin" -- see #5451
                    then withTempDirectory "." templ action
                    else withSystemTempDirectory templ action
       (newCommonState, res) <- liftIO $ withTempDir "tex2pdf." $ \tmpdir' -> do