Fix undefined reference with old versions of openssl

Versions prior to 0.9.8f  did not have this function, rhel/centos5 are still on a
heavily backported version of 0.9.8e and theoretically supported until March 2017

Without this ifdef, I get the following link failure:
```
CMakeFiles/libgit2_clar.dir/src/openssl_stream.c.o: In function `openssl_connect':
openssl_stream.c:(.text+0x45a): undefined reference to `SSL_set_tlsext_host_name'
collect2: error: ld returned 1 exit status
make[6]: *** [libgit2_clar] Error 1
```

We do not want libcurl to perform the TLS negotiation for us, so we
don't need to pass this option.

When linking against libcurl, use it as the underlying transport instead
of straight sockets. We can't quite just give over the file descriptor,
as curl puts it into non-blocking mode, so we build a custom BIO so
OpenSSL sends the data through our stream, be it the socket or curl
streams.

openssl: don't try to handle WANT_READ or WANT_WRITE