shadowsocks-libev: add dependency on thread support

shadowsocks-libev needs thread support, otherwise the configure script
fails with:

configure: error: Can not find pthreads.  This is required.

Signed-off-by: Min Xu <xuminready@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>

package/shadowsocks-libev/Config.in

@@ -1,5 +1,6 @@
 config BR2_PACKAGE_SHADOWSOCKS_LIBEV
 	bool "shadowsocks-libev"
+	depends on BR2_TOOLCHAIN_HAS_THREADS
 	depends on BR2_USE_MMU # fork()
 	select BR2_PACKAGE_C_ARES
 	select BR2_PACKAGE_LIBEV
@@ -11,3 +12,7 @@ config BR2_PACKAGE_SHADOWSOCKS_LIBEV
 	  embedded devices and low-end boxes.
 
 	  https://github.com/shadowsocks/shadowsocks-libev
+
+comment "shadowsocks-libev needs a toolchain w/ threads"
+	depends on BR2_USE_MMU
+	depends on !BR2_TOOLCHAIN_HAS_THREADS