CMake complains The CXX compiler identification is unknown
Your /home/gnu/bin/c++ seem to require additional flag to link things properly and CMake doesn't know about that.
To use /usr/bin/c++ as your compiler run cmake with -DCMAKE_CXX_COMPILER=/usr/bin/c++.
Also, CMAKE_PREFIX_PATH variable sets destination dir where your project' files should be installed. It has nothing to do with CMake installation prefix and CMake itself already know this.
CMake does not find Visual C++ compiler
I have found the solution. While the Visual Studio IDE installed successfully it did not install any build tools and therefore did not install the C++ compiler.
By attempting to manually create a C++ project in the Visual Studio 2015 GUI I was able to prompt it to download the C++ packages. CMake was then able to find the compiler without any difficulty.
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