[XrdTls] Replace "#ifdef HAVE_TLS" with a version-based check

This ensures that the right function is used with both shared and static
libraries, as CMake's check_symbol_exists() doesn't work with static
libraries. The release notes for OpenSSL 1.1.0 mention the deprecation
of the old function and recommends using the new TLS_method(). More
information at https://www.openssl.org/news/cl111.txt.

src/XrdTls/XrdTlsContext.cc

@@ -21,6 +21,7 @@
 #include <openssl/crypto.h>
 #include <openssl/err.h>
 #include <openssl/ssl.h>
+#include <openssl/opensslv.h>
 #include <sys/stat.h>
 
 #include "XrdOuc/XrdOucUtils.hh"
@@ -429,7 +430,7 @@ void Fatal(std::string *eMsg, const char *msg, bool sslmsg=false)
 
 const char *GetTlsMethod(const SSL_METHOD *&meth)
 {
-#ifdef HAVE_TLS
+#if OPENSSL_VERSION_NUMBER > 0x1010000fL /* v1.1.0 */
   meth = TLS_method();
 #else
   meth = SSLv23_method();