flac: fix benchmark linking issue with older glibc versions

Fixes:
http://autobuild.buildroot.net/results/b64/b643824d797050d7c001a3b72598158d8f1c6128/
http://autobuild.buildroot.net/results/a2f/a2f2d7b4e9e7b57bc8691e3ec29ecf818be3f79b/

The benchmark tool uses clock_gettime which is available in librt instead of
libc in older glibc versions.  The build system correctly links with -lrt on
Linux systems, but misdetects non-x86 linux systems as !linux, causing build
failures.

Add a patch to relax the linux OS detection to also work on non-x86 systems.

Patch submitted upstream:
http://lists.xiph.org/pipermail/flac-dev/2017-January/006103.html

Signed-off-by: Peter Korsgaard <peter@korsgaard.com>

package/flac/0001-configure.ac-relax-linux-OS-detection.patch

@@ -0,0 +1,35 @@
+From 8589b44c573d4228a9df3ba7bffdfdf584e7b033 Mon Sep 17 00:00:00 2001
+From: Peter Korsgaard <peter@korsgaard.com>
+Date: Mon, 9 Jan 2017 21:38:29 +0100
+Subject: [PATCH] configure.ac: relax linux OS detection
+
+Not all linux hosts match the *-pc-linux-gnu wildcard, causing build
+failures for older glibc versions where we need to link with -lrt for
+clock_gettime - E.G.:
+
+ - arm-unknown-linux-musleabihf
+ - powerpc-unknown-linux-gnuspe
+ - bfin-linux-linux-uclibc
+ ..
+
+Signed-off-by: Peter Korsgaard <peter@korsgaard.com>
+---
+ configure.ac | 2 +-
+ 1 file changed, 1 insertion(+), 1 deletion(-)
+
+diff --git a/configure.ac b/configure.ac
+index 070ab357..235d2717 100644
+--- a/configure.ac
++++ b/configure.ac
+@@ -186,7 +186,7 @@ esac
+ AM_CONDITIONAL(OS_IS_WINDOWS, test "x$os_is_windows" = xyes)
+ 
+ case "$host" in
+-	*-pc-linux-gnu)
++	*-linux-*)
+ 		sys_linux=true
+ 		AC_DEFINE(FLAC__SYS_LINUX)
+ 		AH_TEMPLATE(FLAC__SYS_LINUX, [define if building for Linux])
+-- 
+2.11.0
+

package/flac/flac.mk

@@ -11,6 +11,8 @@ FLAC_INSTALL_STAGING = YES
 FLAC_DEPENDENCIES = $(if $(BR2_PACKAGE_LIBICONV),libiconv)
 FLAC_LICENSE = Xiph BSD-like (libFLAC), GPLv2+ (tools), LGPLv2.1+ (other libraries)
 FLAC_LICENSE_FILES = COPYING.Xiph COPYING.GPL COPYING.LGPL
+# 0001-configure.ac-relax-linux-OS-detection.patch patches configure.ac
+FLAC_AUTORECONF = YES
 FLAC_CONF_OPTS = \
 	--disable-cpplibs \
 	--disable-xmms-plugin \