Add shebang to update scripts and fix Xcode references
Weak-link against libssh2.

This allows access to the `ssh` transport if you have libssh2 installed (eg. using `brew`, so I'm only looking it `/usr/local/lib`). The libgit2 cmake script will detect it automatically, but since we're building static, it's our responsibility to have it.
Merge pull request #200 from tiennou/script-fix

Add shebang to update scripts and fix Xcode references