macOS Sierra pip install psycopg2 error

pip install psycopg2

错误：

    ld: library not found for -lssl

    clang: error: linker command failed with exit code 1 (use -v to see invocation)

    error: command 'cc' failed with exit status 1

解决方法及缘由：

   mac升级到sierra致使

  $ xcode-select --install

参考连接：https://github.com/brianmario/mysql2/issues/795