Homebrew build doesn't work -- dunno why. I should figure out why and fix it then submit a PR, but I'm lazy.
brew install cmake libtool pkg-config check gengetopt help2man pcsc-lite openssl
git clone https://github.com/Yubico/yubico-piv-tool
cd yubico-piv-tool
mkdir build && cd build
PKG_CONFIG_PATH="/usr/local/opt/openssl@1.1/lib/pkgconfig" cmake ..
make
make install
Then to use it with SSH:
ssh-add -s /usr/local/lib/libykcs11.dylib