Fix pthreads linkage.

config.mk

@@ -254,10 +254,10 @@ ifeq ($(WITH_TLS),yes)
 endif
 
 ifeq ($(WITH_THREADING),yes)
-	LIB_LIBADD:=$(LIB_LIBADD) -lpthread
+	LIB_LDFLAGS:=$(LIB_LDFLAGS) -pthread
 	LIB_CPPFLAGS:=$(LIB_CPPFLAGS) -DWITH_THREADING
 	CLIENT_CPPFLAGS:=$(CLIENT_CPPFLAGS) -DWITH_THREADING
-	STATIC_LIB_DEPS:=$(STATIC_LIB_DEPS) -lpthread
+	STATIC_LIB_DEPS:=$(STATIC_LIB_DEPS) -pthread
 endif
 
 ifeq ($(WITH_SOCKS),yes)