libnl: Fix building with uClibc

uClibc doesn't implement strerror_l() and thus libnl starting from
3.2.29 couldn't be compiled with it any longer, see
6c2d111177

To work-around that problem we'll just do a check on strerror_l()
availability during configuration and if it's not there just fall back
to locale-less strerror().

Patch for libnl is alreadfy merged upstream, see
e15966ac7f
and once the next libnl release happens this one must be removed from
Lede/OpenWrt.

Signed-off-by: Alexey Brodkin <Alexey.Brodkin@synopsys.com>
Cc: Felix Fietkau <nbd@nbd.name>
Cc: John Crispin <john@phrozen.org>
Cc: Daniel Engberg <daniel.engberg.lists@pyret.net>

package/libs/libnl/patches/0001-lib-Escape-usage-of-strerror_l-if-it-doesn-t-exist-i.patch

@@ -0,0 +1,105 @@
+From 098a4cc35b0da4438b8b67a914edecebef5bb6a9 Mon Sep 17 00:00:00 2001
+From: Alexey Brodkin <abrodkin@synopsys.com>
+Date: Fri, 10 Mar 2017 13:22:14 +0300
+Subject: [PATCH] lib: Escape usage of strerror_l() if it doesn't exist in libc
+
+uClibc doesn't implement strerror_l() and thus libnl starting from
+3.2.29 couldn't be compiled with it any longer.
+
+To work-around that problem we'll just do a check on strerror_l()
+availability during configuration and if it's not there just fall back
+to locale-less strerror().
+
+Signed-off-by: Alexey Brodkin <abrodkin@synopsys.com>
+Cc: Andre Draszik <adraszik@tycoint.com>
+Cc: Thomas Haller <thaller@redhat.com>
+---
+
+This patch is now accepted upstream and will be a part of the next libnl
+release, see
+https://github.com/thom311/libnl/commit/e15966ac7f3b43df2acf869f98089762807d0568
+
+ configure.ac    | 2 ++
+ lib/utils.c     | 8 +++++++-
+ src/lib/utils.c | 6 ++++++
+ 3 files changed, 15 insertions(+), 1 deletion(-)
+
+diff --git a/configure.ac b/configure.ac
+index 68b285e5b15c..2739b997ee3a 100644
+--- a/configure.ac
++++ b/configure.ac
+@@ -121,6 +121,8 @@ fi
+ 
+ AC_CONFIG_SUBDIRS([doc])
+ 
++AC_CHECK_FUNCS([strerror_l])
++
+ AC_CONFIG_FILES([
+ Makefile
+ libnl-3.0.pc
+diff --git a/lib/utils.c b/lib/utils.c
+index fb350d13fd2f..06273c5b291e 100644
+--- a/lib/utils.c
++++ b/lib/utils.c
+@@ -30,7 +30,9 @@
+ #include <netlink/utils.h>
+ #include <linux/socket.h>
+ #include <stdlib.h> /* exit() */
++#ifdef HAVE_STRERROR_L
+ #include <locale.h>
++#endif
+ 
+ /**
+  * Global variable indicating the desired level of debugging output.
+@@ -123,9 +125,10 @@ int __nl_read_num_str_file(const char *path, int (*cb)(long, const char *))
+ 
+ const char *nl_strerror_l(int err)
+ {
++	const char *buf;
++#ifdef HAVE_STRERROR_L
+ 	int errno_save = errno;
+ 	locale_t loc = newlocale(LC_MESSAGES_MASK, "", (locale_t)0);
+-	const char *buf;
+ 
+ 	if (loc == (locale_t)0) {
+ 		if (errno == ENOENT)
+@@ -140,6 +143,9 @@ const char *nl_strerror_l(int err)
+ 	}
+ 
+ 	errno = errno_save;
++#else
++	buf = strerror(err);
++#endif
+ 	return buf;
+ }
+ /** @endcond */
+diff --git a/src/lib/utils.c b/src/lib/utils.c
+index 5878f279c364..feb1d4ef4056 100644
+--- a/src/lib/utils.c
++++ b/src/lib/utils.c
+@@ -81,6 +81,7 @@ void nl_cli_fatal(int err, const char *fmt, ...)
+ 		fprintf(stderr, "\n");
+ 	} else {
+ 		char *buf;
++#ifdef HAVE_STRERROR_L
+ 		locale_t loc = newlocale(LC_MESSAGES_MASK, "", (locale_t)0);
+ 		if (loc == (locale_t)0) {
+ 			if (errno == ENOENT)
+@@ -91,9 +92,14 @@ void nl_cli_fatal(int err, const char *fmt, ...)
+ 		}
+ 		if (loc != (locale_t)0)
+ 			buf = strerror_l(err, loc);
++#else
++		buf = strerror(err);
++#endif
+ 		fprintf(stderr, "%s\n", buf);
++#ifdef HAVE_STRERROR_L
+ 		if (loc != (locale_t)0)
+ 			freelocale(loc);
++#endif
+ 	}
+ 
+ 	exit(abs(err));
+-- 
+2.7.4
+