Use #if, not #ifdef with HAVE_ macros

lib/path-concat.c

@@ -1,5 +1,5 @@
 /* path-concat.c -- concatenate two arbitrary pathnames
-   Copyright (C) 1996, 1997 Free Software Foundation, Inc.
+   Copyright (C) 1996, 1997, 1998 Free Software Foundation, Inc.
 
    This program is free software; you can redistribute it and/or modify
    it under the terms of the GNU General Public License as published by
@@ -17,7 +17,7 @@
 
 /* Written by Jim Meyering.  */
 
-#ifdef HAVE_CONFIG_H
+#if HAVE_CONFIG_H
 # include <config.h>
 #endif
 
@@ -26,7 +26,7 @@
 #endif
 
 #include <stdio.h>
-#ifdef HAVE_STRING_H
+#if HAVE_STRING_H
 # include <string.h>
 #endif
 #include <sys/types.h>