Allow multiple tmp-merge directories at once

make -jN was failing because libgrpc and libgrpc++ wanted to link at the same time.
diff --git a/Makefile b/Makefile
index d55d205..1d4708b 100644
--- a/Makefile
+++ b/Makefile
@@ -2566,13 +2566,13 @@
 	$(Q) mkdir -p `dirname $@`
 	$(Q) rm -f $(LIBDIR)/$(CONFIG)/libgrpc.a
 	$(Q) $(AR) rcs $(LIBDIR)/$(CONFIG)/libgrpc.a $(LIBGRPC_OBJS)
-	$(Q) rm -rf tmp-merge
-	$(Q) mkdir tmp-merge
-	$(Q) ( cd tmp-merge ; $(AR) x ../$(LIBDIR)/$(CONFIG)/libgrpc.a )
-	$(Q) for l in $(OPENSSL_MERGE_LIBS) ; do ( cd tmp-merge ; ar x ../$${l} ) ; done
-	$(Q) rm -f $(LIBDIR)/$(CONFIG)/libgrpc.a tmp-merge/__.SYMDEF*
-	$(Q) ar rcs $(LIBDIR)/$(CONFIG)/libgrpc.a tmp-merge/*
-	$(Q) rm -rf tmp-merge
+	$(Q) rm -rf tmp-merge-grpc
+	$(Q) mkdir tmp-merge-grpc
+	$(Q) ( cd tmp-merge-grpc ; $(AR) x ../$(LIBDIR)/$(CONFIG)/libgrpc.a )
+	$(Q) for l in $(OPENSSL_MERGE_LIBS) ; do ( cd tmp-merge-grpc ; ar x ../$${l} ) ; done
+	$(Q) rm -f $(LIBDIR)/$(CONFIG)/libgrpc.a tmp-merge-grpc/__.SYMDEF*
+	$(Q) ar rcs $(LIBDIR)/$(CONFIG)/libgrpc.a tmp-merge-grpc/*
+	$(Q) rm -rf tmp-merge-grpc
 ifeq ($(SYSTEM),Darwin)
 	$(Q) ranlib $(LIBDIR)/$(CONFIG)/libgrpc.a 
 endif
@@ -3141,13 +3141,13 @@
 	$(Q) mkdir -p `dirname $@`
 	$(Q) rm -f $(LIBDIR)/$(CONFIG)/libgrpc++.a
 	$(Q) $(AR) rcs $(LIBDIR)/$(CONFIG)/libgrpc++.a $(LIBGRPC++_OBJS)
-	$(Q) rm -rf tmp-merge
-	$(Q) mkdir tmp-merge
-	$(Q) ( cd tmp-merge ; $(AR) x ../$(LIBDIR)/$(CONFIG)/libgrpc++.a )
-	$(Q) for l in $(OPENSSL_MERGE_LIBS) ; do ( cd tmp-merge ; ar x ../$${l} ) ; done
-	$(Q) rm -f $(LIBDIR)/$(CONFIG)/libgrpc++.a tmp-merge/__.SYMDEF*
-	$(Q) ar rcs $(LIBDIR)/$(CONFIG)/libgrpc++.a tmp-merge/*
-	$(Q) rm -rf tmp-merge
+	$(Q) rm -rf tmp-merge-grpc++
+	$(Q) mkdir tmp-merge-grpc++
+	$(Q) ( cd tmp-merge-grpc++ ; $(AR) x ../$(LIBDIR)/$(CONFIG)/libgrpc++.a )
+	$(Q) for l in $(OPENSSL_MERGE_LIBS) ; do ( cd tmp-merge-grpc++ ; ar x ../$${l} ) ; done
+	$(Q) rm -f $(LIBDIR)/$(CONFIG)/libgrpc++.a tmp-merge-grpc++/__.SYMDEF*
+	$(Q) ar rcs $(LIBDIR)/$(CONFIG)/libgrpc++.a tmp-merge-grpc++/*
+	$(Q) rm -rf tmp-merge-grpc++
 ifeq ($(SYSTEM),Darwin)
 	$(Q) ranlib $(LIBDIR)/$(CONFIG)/libgrpc++.a 
 endif