Successor: Fixed Makefile to enable --std=gnu++11 explicitly

Older versions of GCC (like the one in Debian Jessie) default
to older standards of the language.

01-tree_successor/cpp/Makefile

 test: tree_successor_test
 	./$<
 
-CXXFLAGS=-O2 -Wall -Wextra -g -Wno-sign-compare
+CXXFLAGS=-std=gnu++11 -O2 -Wall -Wextra -g -Wno-sign-compare
 
 tree_successor_test: tree_successor.h tree_successor_test.cpp test_main.cpp
 	$(CXX) $(CXXFLAGS) $^ -o $@