Add a new QT_NO_SSL feature macro.

Add a new QT_NO_SSL feature macro.

At the moment users of Qt must detect if it was compiled with SSL
support by testing for QT_NO_OPENSSL. This means that any code that is
conditionally compiled this way is tied to the presence of the openssl
backend. This commit makes it possible to implement new SSL backends
during the Qt5 lifetime without breaking this code. People can still
test for QT_NO_OPENSSL if they really need openssl, but if they simply
want to know if there's SSL support at all they should use this define
instead.

In addition, this commit changes the public API headers to use the new
define.

Change-Id: Ib57a71aa65836ac9351f120a487bfeb8009d9515
Reviewed-by: Peter Hartmann <peter.hartmann@nokia.com>