[C++] Fix Mac build by using pthread's TLS

Clang on Mac doesn't support thread_local keyword.
diff --git a/Makefile.ckati b/Makefile.ckati
index cd8a0f8..f35824b 100644
--- a/Makefile.ckati
+++ b/Makefile.ckati
@@ -71,7 +71,7 @@
 #KATI_CXXFLAGS += -pg
 
 ifeq ($(shell uname),Linux)
-KATI_LIBS := -lrt
+KATI_LIBS := -lrt -lpthread
 endif
 
 # Rule to build ckati into KATI_BIN_PATH