Solved it. According to https://stackoverflow.com/questions/34654682/unrecognized-command-line-option-stdlib-libc-gcc-homebrew-gcc-5-3-0-5-3-0, -stdlib=libc++ is a flag that’s specific to the clang compiler.
I ran brew install llvm, did some symlinks to make sure the right Clang is called, and now CMake runs smoothly. Thanks!