Fix build script: libssh2 with OpenSSL can be built now with Xcode 5 Command Line Tools

- Change SDKVERSION to 7.0.
- Fix build-libssh2.sh so that libssh with OpenSSL can be built with Xcode 5 (Command Line Tools).
- Update openssl submodule.
Merge pull request #24 from michaelwermeester/master

Fix build script: libssh2 with OpenSSL can be built now with Xcode 5