Where did you do your make clean ?
If you follow the documentation correctly, you will see that it is recommended to create a folder to store everything that will be compiled.
Usushno, you do it by doing
mkdir build
You should not have a problem in the following this guide .
As you can see, cmake command:
cmake -D CMAKE_BUILD_TYPE=RELEASE -D CMAKE_INSTALL_PREFIX=/usr/local -D BUILD_PYTHON_SUPPORT=ON -D BUILD_EXAMPLES=ON ..
The .. at the end is very important because it means you have to execute the command in the directory where you want everything to be built.
If you directly run your cmake in the source directory, you can unzip the package and start over;).
You were right to change the variable for CUDA, especially if you do not have an NVidia graphics card.
The last message you provide is a warning, not an error. And you can solve it by following what I said before.
If you just want to use OpenCV and not be developed for it, you laugh as much as Barnabas, directly using the package manager.
In this case, follows his link.