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

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.

WARNING: It's not possible to build libssh2 with libgcrypt when using this build script. Libgcrypt build script has to be updated first. 

Note: I pulled this request to help those who want to build libssh2 with Xcode Command Line Tools.