The libgit2 shared library must be linked with all the dynamic libraries
it depends (z, openssl).

These libraries are now automatically detected during the configuration
phase.

Also, removed the linking dependency on libgit2 which Scott added: if
you link libgit2 with itself, the universe could implode, and we don't
want that to happen.

Signed-off-by: Vicent Marti <tanoku@gmail.com>