Use GSSAPI library for SSPI auth, when native SSPI is not available

This allows non-Windows clients to connect to a Windows
server with SSPI authentication.

Christian Ullrich, largely modified by me

src/interfaces/libpq/fe-auth.c

@@ -831,6 +831,10 @@ pg_fe_sendauth(AuthRequest areq, PGconn *conn)
 
 #if defined(ENABLE_GSS) || defined(ENABLE_SSPI)
 		case AUTH_REQ_GSS:
+#if !defined(ENABLE_SSPI)
+			/* no native SSPI, so use GSSAPI library for it */
+		case AUTH_REQ_SSPI:
+#endif
 			{
 				int			r;
 
@@ -888,13 +892,14 @@ pg_fe_sendauth(AuthRequest areq, PGconn *conn)
 				pgunlock_thread();
 			}
 			break;
-#else
+#else /* defined(ENABLE_GSS) || defined(ENABLE_SSPI) */
+			/* No GSSAPI *or* SSPI support */
 		case AUTH_REQ_GSS:
 		case AUTH_REQ_GSS_CONT:
 			printfPQExpBuffer(&conn->errorMessage,
 					 libpq_gettext("GSSAPI authentication not supported\n"));
 			return STATUS_ERROR;
-#endif
+#endif /* defined(ENABLE_GSS) || defined(ENABLE_SSPI) */
 
 #ifdef ENABLE_SSPI
 		case AUTH_REQ_SSPI:
@@ -914,11 +919,19 @@ pg_fe_sendauth(AuthRequest areq, PGconn *conn)
 			pgunlock_thread();
 			break;
 #else
+			/*
+			 * No SSPI support. However, if we have GSSAPI but not SSPI
+			 * support, AUTH_REQ_SSPI will have been handled in the codepath
+			 * for AUTH_REQ_GSSAPI above, so don't duplicate the case label
+			 * in that case.
+			 */
+#if !defined(ENABLE_GSS)
 		case AUTH_REQ_SSPI:
 			printfPQExpBuffer(&conn->errorMessage,
 					   libpq_gettext("SSPI authentication not supported\n"));
 			return STATUS_ERROR;
-#endif
+#endif /* !define(ENABLE_GSSAPI) */
+#endif /* ENABLE_SSPI */
 
 
 		case AUTH_REQ_CRYPT: