flush stdout before running subprograms

This is needed to separate Cabal and subprogram output if stdout is
buffered (e.g. a file), especially if stdout and stderr are the same.

Distribution/Simple/Utils.hs

@@ -237,6 +237,7 @@ printRawCommandAndArgs verbosity path args
 rawSystemExit :: Verbosity -> FilePath -> [String] -> IO ()
 rawSystemExit verbosity path args = do
   printRawCommandAndArgs verbosity path args
+  hFlush stdout
   maybeExit $ rawSystem path args
 
 -- Exit with the same exitcode if the subcommand fails