Regardless of the compiler version passed, boost is built with clang++ (no version). You need to use something like https://gist.githubusercontent.com/junkdog/70231d6953592cd6f27def59fe19e50d/raw/92f0e73d2558402b7316021c1ab408b30e534de6/update-alternatives-clang.sh to change the default clang to get a consistent build.
Regardless of the compiler version passed, boost is built with clang++ (no version). You need to use something like https://gist.githubusercontent.com/junkdog/70231d6953592cd6f27def59fe19e50d/raw/92f0e73d2558402b7316021c1ab408b30e534de6/update-alternatives-clang.sh to change the default clang to get a consistent build.