As a performance optimization in the send path, add an AMQP_SF_MORE flag
indicating that more data is intended to be sent, and that packets shouldn't be
sent out on the wire unless there is a full packet's worth of data available.

Use this to specify MSG_MORE to send() on Linux.

Function amqp_ssl_socket_send() should return the number of bytes
written. Retruning AMQP_STATUS_OK on success will cause all
amqp_socket_send() with SSL socket implementation to behave incorrectly.

Autotools build uses c90/gnu90. Add similar functionality for cmake
build. This will work for gcc and clang, while for other compilers the
checks will just fail and no additional flags will be added.

If building with a compiler that does not support C99, this would raise
a warning.

According to documentation SSL_connect() can return:
- 1 if handshake was successful or
- 0 or < 0 if handshake failed and connection was cleaned up or
  communcation error occurred

When nonblocking flag is set on the socket and handshake is in progress,
SSL_connect() retruns -1 and sets error SSL_ERROR_WANT_READ. The test
will then skip the switch() branch, and proceed forward with hanshake
procedure in unfinished state. This may lead to certificate verification
failure, as should the handshake be interrupted too soon, no server
certificates would be received.