Bug: Fix incorrect certtool being called on macOS/Mac OS X
Apple rolls their own certtool on macOS/Mac OS X, which is distinct from the correct gnutls-certtool from gnutls installed by Homebrew/MacPorts. Apple's certtool and GnuTLS' certtool have different command syntaxes, and thus leap cert dh
will fail on macOS in its current implementation. By installing gnutls through Homebrew or MacPorts and creating gnutls-certtool, it can be called when Darwin is detected at Ruby runtime.