Add optional LibSSH2 include and library path in configure.ac
Add optional OpenSSL include and library path in configure.ac
Change URL for bug-report in configure.ac

* Change URL to https://github.com/ropensci/git2r/issues

Signed-off-by: Stefan Widgren <stefan.widgren@gmail.com>
Rebuild configure

Signed-off-by: Stefan Widgren <stefan.widgren@gmail.com>