xargs: avoid sc_useless_cpp_parens failure

$ make syntax-check
...
xargs/xargs.c:674:#if !(defined(SIGUSR1) && defined(SIGUSR2))
maint.mk: found useless parentheses in cpp directive
make: *** [maint.mk:924: sc_useless_cpp_parens] Error 1

* xargs/xargs.c (main): Simplify -P #if conditional to avoid the
above syntax-check failure.

xargs/xargs.c

@@ -671,11 +671,11 @@ main (int argc, char **argv)
 	case 'P':
 	  /* Allow only up to MAX_PROC_MAX child processes. */
 	  proc_max = parse_num (optarg, 'P', 0L, MAX_PROC_MAX, 1);
-#if !(defined(SIGUSR1) && defined(SIGUSR2))
+#if defined SIGUSR1 && defined SIGUSR2
+	  catch_usr_signals = true;
+#else
 	  error (0, 0, _("SIGUSR1 and SIGUSR2 are not both defined, so the -P option does nothing."));
 	  proc_max = 1;
-#else
-	  catch_usr_signals = true;
 #endif
 	  break;