Make the unpleasant sed one-liner less unpleasant

utils/ghc-cabal/ghc.mk

@@ -77,7 +77,7 @@ $(GHC_CABAL_DIR)/dist-dummy-ghc/build/dummy-ghc.hs : $(GHC_CABAL_DIR)/ghc.mk $(M
 #   ["PostfixOperators"] ++
 	echo 'extensions :: [String]'                                     >> $@
 	echo 'extensions ='                                               >> $@
-	sed '/^xFlags/,/]/{/^  (/{s/^[^"]*"/  ["/; s/"[^"]*$$/"] ++/; p}}; d' compiler/main/DynFlags.hs >> $@
+	sed '/^xFlags/,/]/s/^[[:space:]]*([[:space:]]*\("[^"]*"\)[^"]*/  [\1] ++/p;d' compiler/main/DynFlags.hs >> $@
 	echo '  []'                                                       >> $@
 
 # We don't build dummy-ghc with Cabal, so we need to pass -package