use O2 optimize not O3

Author: presburger

CMakeLists.txt

@@ -1,6 +1,6 @@
 cmake_minimum_required(VERSION 2.9)
 project(qmc-decoder)
-set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -O3 -std=c++11")
+set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -O2 -std=c++11")
 aux_source_directory(src SRC)
 add_executable(decoder ${SRC})