Build OpenSSL so Ruby 2.0 installs Just Work on Mountain Lion.

Add support for package-specific configure script and install prefix.

Ruby 2.0 won't compile the OpenSSL extension on Mountain Lion
because Apple shipped a crippled OpenSSL distribution. The fix
is to build your own OpenSSL and link to that instead:

  brew install openssl
  CONFIGURE_OPTS="--with-openssl-dir=`brew --prefix openssl`" \
    rbenv install 2.0.0-rc1

That's a lot of hassle for what will soon be a very common case.
We can fix it by bundling a working OpenSSL.