(General)~extract valgrind memcheck flags to VFLAGS variable

Makefile

@@ -1,16 +1,17 @@
 CC=clang++
 CFLAGS=-pedantic -Wall -ggdb
+VFLAGS=--show-leak-kinds=all --leak-check=full
 
 list: list.cpp
 	$(CC) $(CFLAGS) $^ -o $@
 
 .PHONY: memcheck_list
 memcheck_list: list
-	valgrind --show-leak-kinds=all --leak-check=full ./list;
+	valgrind $(VFLAGS) ./$^;
 
 btree: btree.cpp
 	$(CC) $(CFLAGS) $^ -o $@
 
 .PHONY: memcheck_btree
 memcheck_btree: btree
-	valgrind --show-leak-kinds=all --leak-check=full ./btree;
+	valgrind $(VFLAGS) ./$^;