Make the cgo tool do more linking work

The cgo directives let us do a lot more than I previously thought, so we
can use this to make the building process of git2go go through the go
tool directly rather than via the script.

libgit2 still needs to be built manually, so we do still require make,
but only for building libgit2. Once that's built, any modifications to
git2go's own code can be built with

    go build