Problem: when libgssapi isn't available, GSSAPI options 'seem' to work

Solution: use same approach as for libsodium/CURVE, i.e. return EINVAL
if the library isn't present when libzmq builds, and the application
still tries to use these options in zmq_getsockopt/setsockopt.

src/options.cpp

@@ -404,6 +404,8 @@ int zmq::options_t::setsockopt (int option_, const void *optval_,
             }
             break;
 
+        //  If libgssapi isn't installed, these options provoke EINVAL
+#       ifdef HAVE_LIBGSSAPI_KRB5
         case ZMQ_GSSAPI_SERVER:
             if (is_int && (value == 0 || value == 1)) {
                 as_server = value;
@@ -435,6 +437,7 @@ int zmq::options_t::setsockopt (int option_, const void *optval_,
                 return 0;
             }
             break;
+#       endif
 
         case ZMQ_HANDSHAKE_IVL:
             if (is_int && value >= 0) {
@@ -741,6 +744,8 @@ int zmq::options_t::getsockopt (int option_, void *optval_, size_t *optvallen_)
             }
             break;
  
+        //  If libgssapi isn't installed, these options provoke EINVAL
+#       ifdef HAVE_LIBGSSAPI_KRB5
         case ZMQ_GSSAPI_SERVER:
             if (is_int) {
                 *value = as_server && mechanism == ZMQ_GSSAPI;
@@ -770,6 +775,7 @@ int zmq::options_t::getsockopt (int option_, void *optval_, size_t *optvallen_)
                 return 0;
             }
             break;
+#endif
 
         case ZMQ_HANDSHAKE_IVL:
             if (is_int) {