Add another pkg-config file (libmongoc-ssl-1.0.pc.in) and add to configure.ac.

The pkg-config file should have a dependency on libmongoc-1.0 and add a Cflag of -DMONGOC_ENABLE_SSL (or another well chosen name).

Add to mongoc.h

#ifdef MONGOC_ENABLE_SSL
#include "mongoc-ss.h"
#include "mongoc-stream-tls.h"
#endif