Fix syntax error problem for older versions of GHC

src/compiler/GF/Grammar/Lexer.x

@@ -285,6 +285,10 @@ instance Monad P where
                              POk s a          -> unP (k a) s
                              PFailed posn err -> PFailed posn err
 
+#if !(MIN_VERSION_base(4,13,0))
+  -- Monad(fail) will be removed in GHC 8.8+
+  fail = Fail.fail
+#endif
 
 instance Fail.MonadFail P where
   fail msg    = P $ \(_,AI posn _ _) -> PFailed posn msg