Make the alignof() usage more portable in grep(1)

Remove NetBSD specific check and replace it with GCC extension __alignof__
that is available since GCC 2.x.

Requested by <kre>

external/gpl2/grep/dist/lib/obstack.c

@@ -1,4 +1,4 @@
-/*	$NetBSD: obstack.c,v 1.2 2018/06/13 17:32:29 kamil Exp $	*/
+/*	$NetBSD: obstack.c,v 1.3 2018/06/14 10:14:39 kamil Exp $	*/
 
 /* obstack.c - subroutines used implicitly by object stack macros
    Copyright (C) 1988-1994,96,97,98,99 Free Software Foundation, Inc.
@@ -61,23 +61,7 @@
 
 /* Determine default alignment.  */
 struct fooalign {char x; double d;};
-
-#if defined(__NetBSD__)
-#include <sys/param.h>
-#endif
-
-#ifndef __NetBSD_Prereq__
-#define __NetBSD_Prereq__(a,b,c) 0
-#endif
-
-#if __NetBSD_Prereq__(8,0,0)
-#include <stdalign.h>
-// Avoid Undefined Behavior
-#define DEFAULT_ALIGNMENT ((int)alignof(struct fooalign))
-#else
-#define DEFAULT_ALIGNMENT  \
-  ((PTR_INT_TYPE) ((char *) &((struct fooalign *) 0)->d - (char *) 0))
-#endif
+#define DEFAULT_ALIGNMENT ((int)__alignof__(struct fooalign))
 /* If malloc were really smart, it would round addresses to DEFAULT_ALIGNMENT.
    But in fact it might be less smart and round addresses to as much as
    DEFAULT_ROUNDING.  So we prepare for it to do that.  */