redis: use BR2_TOOLCHAIN_HAS_LIBATOMIC

It uses __atomic_fetch_add_4 so libatomic must be pulled in if
necessary. Fixes:
http://autobuild.buildroot.net/results/dfd/dfdfd77463b0ddd7016202372afcad7fb6fc9ce4/

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

package/redis/Config.in

@@ -1,6 +1,7 @@
 config BR2_PACKAGE_REDIS
 	bool "redis"
 	depends on BR2_USE_MMU # fork()
+	depends on BR2_TOOLCHAIN_HAS_ATOMIC
 	depends on BR2_TOOLCHAIN_HAS_THREADS
 	help
 	  Redis is an open source, advanced key-value store. It is
@@ -11,4 +12,5 @@ config BR2_PACKAGE_REDIS
 
 comment "redis needs a toolchain w/ threads"
 	depends on BR2_USE_MMU
+	depends on BR2_TOOLCHAIN_HAS_ATOMIC
 	depends on !BR2_TOOLCHAIN_HAS_THREADS

package/redis/redis.mk

@@ -13,10 +13,16 @@ define REDIS_USERS
 	redis -1 redis -1 * /var/lib/redis /bin/false - Redis Server
 endef
 
+# Uses __atomic_fetch_add_4
+ifeq ($(BR2_TOOLCHAIN_HAS_LIBATOMIC),y)
+REDIS_LIBATOMIC = -latomic
+endif
+
 # Redis doesn't support DESTDIR (yet, see
 # https://github.com/antirez/redis/pull/609).  We set PREFIX
 # instead.
 REDIS_BUILDOPTS = $(TARGET_CONFIGURE_OPTS) \
+	LDFLAGS="$(TARGET_LDFLAGS) $(REDIS_LIBATOMIC)" \
 	PREFIX=$(TARGET_DIR)/usr MALLOC=libc \
 
 define REDIS_BUILD_CMDS