package/bird: fix build with autoconf >= 2.70

Fix the following build failure raised since bump of autoconf to version
2.71 in commit ecd54b65c1f998a7ccd91f7c523e4ff38c4781da:

checking for glob.h... no
configure: error: glob.h not found.

configure:5207: checking for glob.h
configure:5230: /data/buildroot-autobuilder/instance-0/output-1/host/bin/mips64el-linux-gcc -o conftest -D_LARGEFILE_SOURCE -D_LARGEFILE64_SOURCE -D_FILE_OFFSET_BITS=64  -Os -g0  -pthread -fno-strict-aliasing -fno-strict-overflow -flto -Wall -Wextra -Wstrict-prototypes -Wno-parentheses -Wno-pointer-sign -Wno-missing-field-initializers -D_LARGEFILE_SOURCE -D_LARGEFILE64_SOURCE -D_FILE_OFFSET_BITS=64  -pthread -flto=4 -g conftest.c  >&5
/data/buildroot-autobuilder/instance-0/output-1/host/opt/ext-toolchain/bin/../lib/gcc/mips64el-buildroot-linux-uclibc/8.4.0/../../../../mips64el-buildroot-linux-uclibc/bin/ld: /tmp/ccDboxph.ltrans0.ltrans.o:(.debug_info+0x2a): undefined reference to `conftest.c.0943dc99'
/data/buildroot-autobuilder/instance-0/output-1/host/opt/ext-toolchain/bin/../lib/gcc/mips64el-buildroot-linux-uclibc/8.4.0/../../../../mips64el-buildroot-linux-uclibc/bin/ld: /tmp/ccDboxph.ltrans0.ltrans.o:(.debug_info+0x2f): undefined reference to `conftest.c.0943dc99'

This is the same problem that we tried to fix in
03c148c8de by adding AUTORECONF = YES.

The analysis made back then is probably not entirely correct - the issue
only seems to occur with GCC8, not with any earlier or later version,
which hints at a compiler bug. Since the error involves some
LTO-generated functions, the trigger is probably the -flto=4.

Whatever the exact issue is, it is avoided if we make sure that our
CFLAGS replace the default ones instead of being added to them.
Currently, they are added because the configure script doesn't properly
detect that CFLAGS was set.

Add a patch (sent upstream months ago, but it didn't appear in the
release that has been done since then) that makes sure CFLAGS is
properly detected.

Fixes:
 - http://autobuild.buildroot.org/results/d7aab7df196e0f6d6389b87ff29da7c2971e6b6d

Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>

package/bird/0001-configure.ac-fix-build-with-autoconf-2.70.patch

@@ -0,0 +1,33 @@
+From aa5bc6d2e59c5257d777a45ee4a0e0a784c44244 Mon Sep 17 00:00:00 2001
+From: Fabrice Fontaine <fontaine.fabrice@gmail.com>
+Date: Fri, 5 Nov 2021 20:58:41 +0100
+Subject: [PATCH] configure.ac: properly evaluate ac_test_CFLAGS
+
+Since autoconf 2.69 and
+https://git.savannah.gnu.org/cgit/autoconf.git/commit/?id=76754e04fce5f6a7701bec57b057020585df2ae3
+ac_test_CFLAGS is set to ${CFLAGS+y} instead of ${CFLAGS+set}.
+
+Just test that ac_test_CFLAGS is not empty, to support both cases.
+
+Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
+Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
+---
+ configure.ac | 2 +-
+ 1 file changed, 1 insertion(+), 1 deletion(-)
+
+diff --git a/configure.ac b/configure.ac
+index 64181d29..b4b3fcfb 100644
+--- a/configure.ac
++++ b/configure.ac
+@@ -114,7 +114,7 @@ AC_SEARCH_LIBS([clock_gettime], [rt posix4],
+ AC_CANONICAL_HOST
+ 
+ # Store this value because ac_test_CFLAGS is overwritten by AC_PROG_CC
+-if test "$ac_test_CFLAGS" != set ; then
++if ! test "$ac_test_CFLAGS" ; then
+   bird_cflags_default=yes
+ fi
+ 
+-- 
+2.33.0
+

package/bird/bird.mk

@@ -11,6 +11,8 @@ BIRD_LICENSE_FILES = README
 BIRD_CPE_ID_VENDOR = nic
 BIRD_SELINUX_MODULES = bird
 BIRD_DEPENDENCIES = host-flex host-bison
+
+# 0001-configure.ac-fix-build-with-autoconf-2.70.patch
 BIRD_AUTORECONF = YES
 
 ifeq ($(BR2_PACKAGE_BIRD_CLIENT),y)