janus-gateway: add a patch to use -Wunused-but-set-variable only when available

Fixes:

  http://autobuild.buildroot.org/results/3f0/3f07574e6e4edda9e31fcb0de520a4dbabe6b94a/

[Thomas:
 - Improved configure.ac logic, as suggested by Yann E. Morin.
 - Added a comment in the .mk file to indicate why we're using
   AUTORECONF = YES. Suggested by Yann as well.]

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>

2 files changed, 135 insertions, 1 deletions

diff --git a/package/janus-gateway/0002-Add-test-for-Wunused-but-set-variable.patch b/package/janus-gateway/0002-Add-test-for-Wunused-but-set-variable.patch
new file mode 100644
index 000000000..14acae557
--- /dev/null
+++ b/package/janus-gateway/0002-Add-test-for-Wunused-but-set-variable.patch
@@ -0,0 +1,133 @@
+From c0f075bdee5edabf8f19b68e0880fbd249a89653 Mon Sep 17 00:00:00 2001
+From: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
+Date: Sat, 3 Jan 2015 15:09:59 +0100
+Subject: [PATCH] Add test for -Wunused-but-set-variable
+
+The -Wunused-but-set-variable option does not exist in some old gcc
+versions (gcc 4.5.x), so using it unconditionally breaks the build
+with such compilers.
+
+This commit introduces the AX_CHECK_COMPILE_FLAG m4 macro taken from
+the autoconf-archive
+(http://git.savannah.gnu.org/gitweb/?p=autoconf-archive.git;a=blob_plain;f=m4/ax_check_compile_flag.m4),
+and uses it to detect if the -Wunused-but-set-variable option is
+supported, and only uses it in this case.
+
+Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
+---
+ Makefile.am                 |  2 +-
+ configure.ac                |  3 ++
+ m4/ax_check_compile_flag.m4 | 74 +++++++++++++++++++++++++++++++++++++++++++++
+ 3 files changed, 78 insertions(+), 1 deletion(-)
+ create mode 100644 m4/ax_check_compile_flag.m4
+
+diff --git a/Makefile.am b/Makefile.am
+index 5d23e0b..0f51f6a 100644
+--- a/Makefile.am
++++ b/Makefile.am
+@@ -6,7 +6,7 @@ AM_CFLAGS = \
+ 	-Wnested-externs -Wstrict-prototypes -Wunsafe-loop-optimizations \
+ 	-Wwrite-strings -Wno-missing-field-initializers -Wno-unused-parameter \
+ 	-Wcast-align -Wformat-nonliteral -Wformat-security -Wswitch-default \
+-	-Wmissing-include-dirs -Waggregate-return -Wunused-but-set-variable \
++	-Wmissing-include-dirs -Waggregate-return @GCC_WARN_UNUSED_BUT_SET@ \
+ 	-Warray-bounds -Wold-style-definition -Wsign-compare -Wlarger-than=65537
+ AM_CFLAGS += -Wno-undef  # sophiasip is not -Wundef--safe
+ AM_CFLAGS += -Wredundant-decls  # sophiasip also contains redundant declarations
+diff --git a/configure.ac b/configure.ac
+index 17870a7..f9bb1e4 100644
+--- a/configure.ac
++++ b/configure.ac
+@@ -7,6 +7,9 @@ AM_SILENT_RULES([yes])
+ AC_GNU_SOURCE
+ 
+ AC_PROG_CC
++AX_CHECK_COMPILE_FLAG([-Wunused-but-set-variable],
++	[GCC_WARN_UNUSED_BUT_SET=-Wunused-but-set-variable])
++AC_SUBST(GCC_WARN_UNUSED_BUT_SET)
+ 
+ LT_PREREQ([2.2])
+ LT_INIT
+diff --git a/m4/ax_check_compile_flag.m4 b/m4/ax_check_compile_flag.m4
+new file mode 100644
+index 0000000..51df0c0
+--- /dev/null
++++ b/m4/ax_check_compile_flag.m4
+@@ -0,0 +1,74 @@
++# ===========================================================================
++#   http://www.gnu.org/software/autoconf-archive/ax_check_compile_flag.html
++# ===========================================================================
++#
++# SYNOPSIS
++#
++#   AX_CHECK_COMPILE_FLAG(FLAG, [ACTION-SUCCESS], [ACTION-FAILURE], [EXTRA-FLAGS], [INPUT])
++#
++# DESCRIPTION
++#
++#   Check whether the given FLAG works with the current language's compiler
++#   or gives an error.  (Warnings, however, are ignored)
++#
++#   ACTION-SUCCESS/ACTION-FAILURE are shell commands to execute on
++#   success/failure.
++#
++#   If EXTRA-FLAGS is defined, it is added to the current language's default
++#   flags (e.g. CFLAGS) when the check is done.  The check is thus made with
++#   the flags: "CFLAGS EXTRA-FLAGS FLAG".  This can for example be used to
++#   force the compiler to issue an error when a bad flag is given.
++#
++#   INPUT gives an alternative input source to AC_COMPILE_IFELSE.
++#
++#   NOTE: Implementation based on AX_CFLAGS_GCC_OPTION. Please keep this
++#   macro in sync with AX_CHECK_{PREPROC,LINK}_FLAG.
++#
++# LICENSE
++#
++#   Copyright (c) 2008 Guido U. Draheim <guidod@gmx.de>
++#   Copyright (c) 2011 Maarten Bosmans <mkbosmans@gmail.com>
++#
++#   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 the
++#   Free Software Foundation, either version 3 of the License, or (at your
++#   option) any later version.
++#
++#   This program is distributed in the hope that it will be useful, but
++#   WITHOUT ANY WARRANTY; without even the implied warranty of
++#   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU General
++#   Public License for more details.
++#
++#   You should have received a copy of the GNU General Public License along
++#   with this program. If not, see <http://www.gnu.org/licenses/>.
++#
++#   As a special exception, the respective Autoconf Macro's copyright owner
++#   gives unlimited permission to copy, distribute and modify the configure
++#   scripts that are the output of Autoconf when processing the Macro. You
++#   need not follow the terms of the GNU General Public License when using
++#   or distributing such scripts, even though portions of the text of the
++#   Macro appear in them. The GNU General Public License (GPL) does govern
++#   all other use of the material that constitutes the Autoconf Macro.
++#
++#   This special exception to the GPL applies to versions of the Autoconf
++#   Macro released by the Autoconf Archive. When you make and distribute a
++#   modified version of the Autoconf Macro, you may extend this special
++#   exception to the GPL to apply to your modified version as well.
++
++#serial 3
++
++AC_DEFUN([AX_CHECK_COMPILE_FLAG],
++[AC_PREREQ(2.59)dnl for _AC_LANG_PREFIX
++AS_VAR_PUSHDEF([CACHEVAR],[ax_cv_check_[]_AC_LANG_ABBREV[]flags_$4_$1])dnl
++AC_CACHE_CHECK([whether _AC_LANG compiler accepts $1], CACHEVAR, [
++  ax_check_save_flags=$[]_AC_LANG_PREFIX[]FLAGS
++  _AC_LANG_PREFIX[]FLAGS="$[]_AC_LANG_PREFIX[]FLAGS $4 $1"
++  AC_COMPILE_IFELSE([m4_default([$5],[AC_LANG_PROGRAM()])],
++    [AS_VAR_SET(CACHEVAR,[yes])],
++    [AS_VAR_SET(CACHEVAR,[no])])
++  _AC_LANG_PREFIX[]FLAGS=$ax_check_save_flags])
++AS_IF([test x"AS_VAR_GET(CACHEVAR)" = xyes],
++  [m4_default([$2], :)],
++  [m4_default([$3], :)])
++AS_VAR_POPDEF([CACHEVAR])dnl
++])dnl AX_CHECK_COMPILE_FLAGS
+-- 
+2.1.0
+
diff --git a/package/janus-gateway/janus-gateway.mk b/package/janus-gateway/janus-gateway.mk
index 6c89e9a0f..d2556e0c2 100644
--- a/package/janus-gateway/janus-gateway.mk
+++ b/package/janus-gateway/janus-gateway.mk
@@ -13,7 +13,8 @@ JANUS_GATEWAY_LICENSE_FILES = COPYING
 JANUS_GATEWAY_DEPENDENCIES = host-pkgconf libmicrohttpd jansson \
 	libnice sofia-sip libsrtp host-gengetopt openssl ding-libs
 
-# Straight out of the repository, no ./configure
+# Straight out of the repository, no ./configure, and we also patch
+# configure.ac.
 JANUS_GATEWAY_AUTORECONF = YES
 
 define JANUS_GATEWAY_M4