I don’t have any of my Linux environments with me at the moment, but have you tried this stackoverflow answer on 32bit compilation on a 64bit machine with gcc/cmake ?
Alternatively, if you use add_subdirectory( glfw ) to include the glfw cmake project in your projects cmake file then I think the build flag -m32 should carry over. Note you need glfw to be a subdir of your project above where you have the cmakelists.txt file.