Support Rubinius linking against Homebrew openssl

Internally, ruby-build will want to set `--with-openssl-dir` if we try
to detect Homebrew's OpenSSL or build one for the OS X. However,
Rubinius doesn't support that configure flag. So, this translates
`--with-openssl-dir` to a combination of `--with-lib-dir` and
`--with-include-dir`.