Building on Ubuntu 16, missing SSL during ./configure
I fixed the issue by installing extra libraries, the whole list

sudo apt-get install -y autoconf g++ make openssl libssl-dev libcurl4-openssl-dev

sudo apt-get install -y libcurl4-openssl-dev pkg-config

sudo apt-get install -y libsasl2-dev


Source: https://github.com/mongodb/mongo-php-driver/issues/138#issuecomment-184749966