Install git2r from source on MacOSX Sierra

Install homebrew

In terminal:

brew install wget
brew install openssl
brew install libssh2

cd ~/Downloads/

wget https://cran.r-project.org/src/contrib/git2r_0.21.0.tar.gz

R CMD INSTALL --configure-args='--with-libssl-include=/opt/local/include --with- 
           libssl-lib=/opt/local/lib' git2r_0.21.0.tar.gz

rm git2r_0.21.0.tar.gz