How to compile without optimizations -O0 using CMake?

I am using scientific linux. I am trying to compile a project that uses a bunch of cpp files.

In the directory user/project/Build, I enter make to compile and link all the cpp files. I then have to go to user/run/ and then type ./run.sh values.txt

To debug with gdb, I have to go to user/run and then type gdb ../project/Build/bin/Project and to run, I enter run -Project INPUT/inputfile.txt. However, I am trying to print out the value of variable using p variablename. However, I get the message s1 = <value optimized out>. I done some research online, and it seems I need to compile without optimizations using -O0 to resolve this. But where do I enter that? In the CMakeLists? If so, which CMakeLists? The one in project/Build or project/src/project?