btrfs-progs: fix build breakage on uClibc without backtrace

uClibc has optional support for backtrace() hence on default
configurations where it's disabled it'll fail.
Add patch to make backtrace support conditional and disable it for
uClibc-based builds since it's mostly for debugging purposes. Fixes:
http://autobuild.buildroot.net/results/325/32523398a8c66a7ac6d3e789332d1b57e090aef1/

Patch status: sent upstream.

Signed-off-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>

package/btrfs-progs/btrfs-progs-0001-add-option-to-disable-backtrace-usage.patch

@@ -0,0 +1,88 @@
+From eb8d1bbdfea80a50ce9fbf3238062a543036f855 Mon Sep 17 00:00:00 2001
+From: Gustavo Zacarias <gustavo@zacarias.com.ar>
+Date: Tue, 7 Oct 2014 11:03:03 -0300
+Subject: [PATCH] btrfs-progs: add option to disable backtrace usage
+
+This commit adds the support for a make variable named
+"DISABLE_BACKTRACE" which allows to disable the support for backtrace()
+usage on ASSERT(), BUG() and BUG_ON() calls.
+This is useful because some alternative C libraries like uClibc have
+optional support for backtrace() which is rarely built when debugging
+isn't taking place.
+
+Signed-off-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
+---
+ Makefile     |  4 ++++
+ kerncompat.h | 15 +++++++++++++++
+ 2 files changed, 19 insertions(+)
+
+diff --git a/Makefile b/Makefile
+index 7cc7783..03a4779 100644
+--- a/Makefile
++++ b/Makefile
+@@ -63,6 +63,10 @@ BUILDDIRS = $(patsubst %,build-%,$(SUBDIRS))
+ INSTALLDIRS = $(patsubst %,install-%,$(SUBDIRS))
+ CLEANDIRS = $(patsubst %,clean-%,$(SUBDIRS))
+ 
++ifeq ($(DISABLE_BACKTRACE),1)
++AM_CFLAGS += -DBTRFS_DISABLE_BACKTRACE
++endif
++
+ ifneq ($(DISABLE_DOCUMENTATION),1)
+ BUILDDIRS += build-Documentation
+ INSTALLDIRS += install-Documentation
+diff --git a/kerncompat.h b/kerncompat.h
+index 19c7fa5..889d94c 100644
+--- a/kerncompat.h
++++ b/kerncompat.h
+@@ -29,7 +29,9 @@
+ #include <stddef.h>
+ #include <linux/types.h>
+ #include <stdint.h>
++#ifndef BTRFS_DISABLE_BACKTRACE
+ #include <execinfo.h>
++#endif
+ 
+ #define ptr_to_u64(x)	((u64)(uintptr_t)x)
+ #define u64_to_ptr(x)	((void *)(uintptr_t)x)
+@@ -55,6 +57,7 @@
+ #define ULONG_MAX       (~0UL)
+ #endif
+ 
++#ifndef BTRFS_DISABLE_BACKTRACE
+ #define MAX_BACKTRACE	16
+ static inline void print_trace(void)
+ {
+@@ -81,6 +84,9 @@ static inline void assert_trace(const char *assertion, const char *filename,
+ }
+ 
+ #define BUG() assert_trace(NULL, __FILE__, __func__, __LINE__, 0)
++#else
++#define BUG() assert(0)
++#endif
+ 
+ #ifdef __CHECKER__
+ #define __force    __attribute__((force))
+@@ -264,10 +270,19 @@ static inline long IS_ERR(const void *ptr)
+ #define kstrdup(x, y) strdup(x)
+ #define kfree(x) free(x)
+ 
++#ifndef BTRFS_DISABLE_BACKTRACE
+ #define BUG_ON(c) assert_trace(#c, __FILE__, __func__, __LINE__, !(c))
++#else
++#define BUG_ON(c) assert(!(c))
++#endif
+ 
+ #define WARN_ON(c) BUG_ON(c)
++
++#ifndef BTRFS_DISABLE_BACKTRACE
+ #define	ASSERT(c) assert_trace(#c, __FILE__, __func__, __LINE__, (c))
++#else
++#define ASSERT(c) assert(c)
++#endif
+ 
+ #define container_of(ptr, type, member) ({                      \
+         const typeof( ((type *)0)->member ) *__mptr = (ptr);    \
+-- 
+2.0.4
+

package/btrfs-progs/btrfs-progs.mk

@@ -8,10 +8,11 @@ BTRFS_PROGS_VERSION = 3.16.2
 BTRFS_PROGS_SITE = https://www.kernel.org/pub/linux/kernel/people/kdave/btrfs-progs
 BTRFS_PROGS_SOURCE = btrfs-progs-v$(BTRFS_PROGS_VERSION).tar.xz
 BTRFS_PROGS_DEPENDENCIES = acl attr e2fsprogs lzo util-linux zlib
+BTRFS_PROGS_MAKE_FLAGS = DISABLE_DOCUMENTATION=1 \
+	$(if $(BR2_TOOLCHAIN_USES_UCLIBC),DISABLE_BACKTRACE=1)
 BTRFS_PROGS_LICENSE = GPLv2
 BTRFS_PROGS_LICENSE_FILES = COPYING
 
-BTRFS_PROGS_MAKE_FLAGS = DISABLE_DOCUMENTATION=1
 
 ifeq ($(BR2_PREFER_STATIC_LIB),y)
 BTRFS_PROGS_MAKE_TARGET = static