Fix for warning issue in libtool

Reference:
http://lists.gnu.org/archive/html/automake/2012-05/msg00014.html

Re: automake 1.12 and AM_PROG_AR

From:	 Nick Bowler
Subject:	 Re: automake 1.12 and AM_PROG_AR
Date:	 Thu, 10 May 2012 17:16:04 -0400
User-agent:	 Mutt/1.5.21 (2010-09-15)
On 2012-05-10 12:52 +0200, Simon Josefsson wrote:
> Hi,
>
> I have received patches similar to the one below for several of my
> projects already, and I'm beginning to think there may be something that
> could be improved in automake.  Why does automake 1.12 need something
> that 1.11 doesn't even recognize?  Perhaps a smoother way to introduce
> this behaviour is to let 1.12 automatically enable it?
>
> Alternatively, is there some other way to address the warnings that is
> better?  Does it indicate a real problem in my projects?
>
> http://lists.gnu.org/archive/html/help-libidn/2012-05/msg00000.html
> http://lists.gnu.org/archive/html/help-libidn/2012-05/txtqAGOGgtPqj.txt

FYI, since the patch is so short it would have been helpful to include
it inline in your email.  I've taken the liberty of reproducing it
below:

> --- libidn-1.24.orig/configure.ac
> +++ libidn-1.24/configure.ac
> @@ -34,6 +34,10 @@ AC_SUBST(LT_CURRENT, 17)
>  AC_SUBST(LT_REVISION, 7)
>  AC_SUBST(LT_AGE, 6)
>
> +# automake 1.12 seems to require this, but automake 1.11 doesn't recognize it
> +m4_pattern_allow([AM_PROG_AR])
> +AM_PROG_AR
> +
>  AC_PROG_CC
>  gl_EARLY
>  lgl_EARLY

First off, this patch seems wrong at a first glance by its use of
m4_pattern_allow: this is saying that "It's ok for AM_PROG_AR to appear
verbatim in the configure script".  This suggests to me that the patch
didn't actually get tested (well) on Automake 1.11, since there will
almost certainly be an error about AM_PROG_AR at configure time.

This one should actually work properly on both old and new Automake
(untested):

  m4_ifdef([AM_PROG_AR], [AM_PROG_AR])

That being said, the new warning is enabled by the -Wextra-portability
option that was added to Automake-1.12.  I'm by no means an export on
this issue, but I think it relates to new support for building with the
Microsoft compiler: in other words, I think that ignoring this warning
will not cause regressions in your package but, instead, your package
will simply not be able to make use of the new feature.

Regardless, I believe that for most packages the new warning is actually
caused by Libtool, and therefore the correct place to add the call to
AM_PROG_AR is in libtool itself, rather than ad-hoc patching of each
package individually.  You can silence the warnings (but not fix them)
by adding -Wno-extra-portability to AM_INIT_AUTOMAKE.

Cheers,
--
Nick Bowler, Elliptic Technologies (http://www.elliptictech.com/)
(cherry picked from commit 350999187285300b1b7dec574cc745617e239bec)

configure.ac

@@ -518,6 +518,7 @@ AC_MSG_RESULT($docdir)
 #
 AC_PROG_CC
 AM_PROG_CC_C_O
+m4_ifdef([AM_PROG_AR], [AM_PROG_AR])
 AC_PROG_LIBTOOL
 AC_PROG_INSTALL
 AC_PROG_MAKE_SET