[PATCH] include_next: silence autoconf 2.68 warning

[Eric Blake]

Next warning spotted by Autoconf; this time it is spurious (when
checking for a working include_next, we could care less what other
defines have already been provided by AC_DEFINE).  Which makes this the
first real-world use case outside of autoconf guts that can reasonably
use the new autoconf macro.

* m4/include_next.m4 (gl_INCLUDE_NEXT): Mark this use of
AC_COMPILE_IFELSE as special.
(AC_LANG_DEFINES_PROVIDED): Provide dummy implementation for
autoconf < 2.68.

Signed-off-by: Eric Blake <address@hidden>
---
 ChangeLog          |    6 ++++++
 m4/include_next.m4 |    9 +++++++--
 2 files changed, 13 insertions(+), 2 deletions(-)

diff --git a/ChangeLog b/ChangeLog
index 5ae6823..146e522 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,5 +1,11 @@
 2010-08-26  Eric Blake  <address@hidden>

+       include_next: silence autoconf 2.68 warning
+       * m4/include_next.m4 (gl_INCLUDE_NEXT): Mark this use of
+       AC_COMPILE_IFELSE as special.
+       (AC_LANG_DEFINES_PROVIDED): Provide dummy implementation for
+       autoconf < 2.68.
+
        acl: fix compilation test
        * m4/acl.m4 (gl_FUNC_ALL): Use correct format for
        AC_COMPILE_IFELSE.
diff --git a/m4/include_next.m4 b/m4/include_next.m4
index c7e0672..a90c488 100644
--- a/m4/include_next.m4
+++ b/m4/include_next.m4
@@ -68,10 +68,11 @@ EOF
 EOF
      gl_save_CPPFLAGS="$CPPFLAGS"
      CPPFLAGS="$gl_save_CPPFLAGS -Iconftestd1b -Iconftestd2"
-     AC_COMPILE_IFELSE([#include <conftest.h>],
+dnl We intentionally avoid using AC_LANG_SOURCE here.
+     AC_COMPILE_IFELSE([AC_LANG_DEFINES_PROVIDED[#include <conftest.h>]],
        [gl_cv_have_include_next=yes],
        [CPPFLAGS="$gl_save_CPPFLAGS -Iconftestd1a -Iconftestd2"
-        AC_COMPILE_IFELSE([#include <conftest.h>],
+        AC_COMPILE_IFELSE([AC_LANG_DEFINES_PROVIDED[#include <conftest.h>]],
           [gl_cv_have_include_next=buggy],
           [gl_cv_have_include_next=no])
        ])
@@ -185,3 +186,7 @@ AC_DEFUN([gl_CHECK_NEXT_HEADERS],
        [$gl_next_as_first_directive])
      AS_VAR_POPDEF([gl_next_header])])
 ])
+
+# Autoconf 2.68 added warnings for our use of AC_COMPILE_IFELSE;
+# this fallback is safe for all earlier autoconf versions.
+m4_define_default([AC_LANG_DEFINES_PROVIDED])
-- 
1.7.2.2