realpath don't works on macOS

Hi @tarsius With latest version of Borg,`make bootstrap-borg` don't works on macOS anymore, will get the errors as below. This is because the realpath on macOS is not `grealpath`. ``` > make bootstrap-borg realpath: illegal option -- - usage: realpath [-q] [path ...] mkdir: /modules: Read-only file system Cloning into 'lib/borg'... fatal: Invalid path '/modules': No such file or directory make: *** [bootstrap-borg] Error 128 ```