libunwind: needs threads

Fixes:
http://autobuild.buildroot.net/results/39a/39a32d323252e3d986fd9290108db7a496d429d9/

Signed-off-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>

package/libunwind/Config.in

@@ -1,8 +1,12 @@
 config BR2_PACKAGE_LIBUNWIND
 	bool "libunwind"
+	depends on BR2_TOOLCHAIN_HAS_THREADS
 	depends on BR2_arm || BR2_armeb || BR2_mips || BR2_mipsel || BR2_mips64 || \
 		BR2_mips64el || BR2_powerpc || BR2_sh || BR2_sh64 || BR2_i386 || BR2_x86_64
 	help
 	  C API to determine the call-chain of a program.
 
 	  http://www.nongnu.org/libunwind/index.html
+
+comment "libunwind requires a toolchain with thread support"
+	depends on !BR2_TOOLCHAIN_HAS_THREADS