Feature-flag TLS library for now.

We'll need to update the configure script to check for OpenSSL and conditionally build this. Ekam users will need to add -DKJ_HAS_OPENSSL to their CXXFLAGS if they've arranged to link against the correct libs.

c++/src/kj/compat/tls-test.c++

@@ -19,6 +19,8 @@
 // OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
 // THE SOFTWARE.
 
+#if KJ_HAS_OPENSSL
+
 #include "tls.h"
 #include <kj/test.h>
 #include <kj/async-io.h>
@@ -438,3 +440,5 @@ KJ_TEST("TLS to capnproto.org") {
 
 }  // namespace
 }  // namespace kj
+
+#endif  // KJ_HAS_OPENSSL

c++/src/kj/compat/tls.c++

@@ -19,6 +19,8 @@
 // OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
 // THE SOFTWARE.
 
+#if KJ_HAS_OPENSSL
+
 #include "tls.h"
 #include "readiness-io.h"
 #include <openssl/bio.h>
@@ -767,3 +769,4 @@ TlsCertificate::~TlsCertificate() noexcept(false) {
 
 }  // namespace kj
 
+#endif  // KJ_HAS_OPENSSL