Build another OpenCV 3 library for a different Python interpreter in the same computer

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I am using Mac and already installed a computer vision library, OpenCV 3 for my computer (system-wide) and it is compiled with support for Python 2 interpreter. I am now, want to build another OpenCV 3 library, reusing that same git clone of OpenCV, that will be compiled for the Python 3 interpreter. I want to build/install it locally. Before I do anything, I want to confirm if my step would be right since I want to prevent any file tanglation.

1. It is said that I have to delete the CMakeCache.txt file inside the build folder of my previous build in order to build the new OpenCV. Is it true? I mean, my previous build folder (for Python 2) is inside the repo folder such that ~/opencv/build, but shouldn't it be just fine?
2. I know I have to set up the flag CMAKE_INSTALL_PREFIX to somewhere else, but as long as I do not make install, it should be just fine, isn't? Since I believe that flag is just to set up the make install target.