--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,21 @@
+2011-01-18  Paul Eggert  <eggert@cs.ucla.edu>
+
+	include_next: do not check for standard headers like stddef.h
+
+	I found this problem when modifying Emacs to use gnulib.
+	I noticed that it added HAVE_STDDEF_H to config.h, even though
+	gnulib always assumes <stddef.h> exists as per README and this
+	symbol is unnecessary.
+	* m4/include_next.m4 (gl_NEXT_HEADERS): New macro, which does not
+	use AC_CHECK_HEADERS_ONCE, but which otherwise contains what
+	gl_CHECK_NEXT_HEADERS used to contain.  This makes 'configure' run
+	faster for headers like stddef.h that are known to exist.
+	(gl_CHECK_NEXT_HEADERS): Use it.
+	* m4/float_h.m4 (gl_FLOAT_H): For float.h, use gl_NEXT_HEADERS
+	rather than gl_CHECK_NEXT_HEADERS.
+	* m4/stdarg.m4 (gl_STDARG_H): Likewise, for stdarg.h.
+	* m4/stddef_h.m4 (gl_STDDEF_H): Likewise, for stddef.h.
+
 2011-01-18  Eric Blake  <eblake@redhat.com>
 
 	ansi-c++-opt: skip C++ dependency style if C++ is unused

--- a/m4/float_h.m4
+++ b/m4/float_h.m4
@@ -1,4 +1,4 @@
-# float_h.m4 serial 4
+# float_h.m4 serial 5
 dnl Copyright (C) 2007, 2009-2011 Free Software Foundation, Inc.
 dnl This file is free software; the Free Software Foundation
 dnl gives unlimited permission to copy and/or distribute it,
@@ -12,7 +12,7 @@
   case "$host_os" in
     beos* | openbsd* | mirbsd*)
       FLOAT_H=float.h
-      gl_CHECK_NEXT_HEADERS([float.h])
+      gl_NEXT_HEADERS([float.h])
       ;;
   esac
   AC_SUBST([FLOAT_H])

--- a/m4/include_next.m4
+++ b/m4/include_next.m4
@@ -1,4 +1,4 @@
-# include_next.m4 serial 16
+# include_next.m4 serial 17
 dnl Copyright (C) 2006-2011 Free Software Foundation, Inc.
 dnl This file is free software; the Free Software Foundation
 dnl gives unlimited permission to copy and/or distribute it,
@@ -149,9 +149,19 @@
 # provides some type/enum definitions or function/variable declarations.
 AC_DEFUN([gl_CHECK_NEXT_HEADERS],
 [
+  AC_CHECK_HEADERS_ONCE([$1])
+  gl_NEXT_HEADERS([$1])
+])
+
+# gl_NEXT_HEADERS(HEADER1 HEADER2 ...)
+# ------------------------------------
+# Like gl_CHECK_NEXT_HEADERS, except do not check whether the headers exist.
+# This is suitable for headers like <stddef.h> that are standardized by C89
+# and therefore can be assumed to exist.
+AC_DEFUN([gl_NEXT_HEADERS],
+[
   AC_REQUIRE([gl_INCLUDE_NEXT])
   AC_REQUIRE([AC_CANONICAL_HOST])
-  AC_CHECK_HEADERS_ONCE([$1])
 
   m4_foreach_w([gl_HEADER_NAME], [$1],
     [AS_VAR_PUSHDEF([gl_next_header],

--- a/m4/stdarg.m4
+++ b/m4/stdarg.m4
@@ -1,4 +1,4 @@
-# stdarg.m4 serial 4
+# stdarg.m4 serial 5
 dnl Copyright (C) 2006, 2008-2011 Free Software Foundation, Inc.
 dnl This file is free software; the Free Software Foundation
 dnl gives unlimited permission to copy and/or distribute it,
@@ -41,7 +41,7 @@
     if test $gl_aixcc = yes; then
       dnl Provide a substitute <stdarg.h> file.
       STDARG_H=stdarg.h
-      gl_CHECK_NEXT_HEADERS([stdarg.h])
+      gl_NEXT_HEADERS([stdarg.h])
       dnl Fallback for the case when <stdarg.h> contains only macro definitions.
       if test "$gl_cv_next_stdarg_h" = '""'; then
         gl_cv_next_stdarg_h='"///usr/include/stdarg.h"'

--- a/m4/stddef_h.m4
+++ b/m4/stddef_h.m4
@@ -1,5 +1,5 @@
 dnl A placeholder for POSIX 2008 <stddef.h>, for platforms that have issues.
-# stddef_h.m4 serial 2
+# stddef_h.m4 serial 3
 dnl Copyright (C) 2009-2011 Free Software Foundation, Inc.
 dnl This file is free software; the Free Software Foundation
 dnl gives unlimited permission to copy and/or distribute it,
@@ -25,7 +25,7 @@
     STDDEF_H=stddef.h
   fi
   if test -n "$STDDEF_H"; then
-    gl_CHECK_NEXT_HEADERS([stddef.h])
+    gl_NEXT_HEADERS([stddef.h])
   fi
 ])