Makefile: add prefix_install target.

Add a Makefile target to do a more normal Unix-style prefix install. For the Homebrew formula at https://github.com/Homebrew/homebrew/pull/34807.

Personally I think this should be the default behaviour of `make install` as it's more expected for Unixy stuff but up to you here. Also, if some of this logic should be in Xcode/bootstrap/elsewhere just let me know.