Add -DNDEBUG flag to compilation options

This will speed up execution as it will prevent from testing the asserts

Makefile

 CXX=g++
-CXXFLAGS=-O2 -Wall -g
+CXXFLAGS=-O2 -Wall -g -DNDEBUG
 SRC=$(wildcard *.cpp)
 OBJ=$(SRC:.cpp=.o)
 EXE=$(SRC:.cpp=)