Further adjust a libssh2 preprocessor function-symbol definition check

configure.ac

@@ -1476,10 +1476,6 @@ if test X"$OPT_LIBSSH2" != Xno; then
     LIBSSH2_ENABLED=1
     AC_DEFINE(USE_LIBSSH2, 1, [if libSSH2 is in use])
     AC_SUBST(USE_LIBSSH2, [1])
-
-    dnl check for this function only present in libssh2 0.19+
-    AC_CHECK_FUNCS( libssh2_session_block_directions )
-
   )
 
   if test X"$OPT_LIBSSH2" != Xoff &&

lib/ssh.h

@@ -30,9 +30,9 @@
 #endif
 
 #if (LIBSSH2_VERSION_NUM >= 0x001300)
-#  ifndef HAVE_LIBSSH2_SESSION_BLOCK_DIRECTIONS
-#    define HAVE_LIBSSH2_SESSION_BLOCK_DIRECTIONS 1
-#  endif
+#  define HAVE_LIBSSH2_SESSION_BLOCK_DIRECTIONS 1
+#else
+#  undef HAVE_LIBSSH2_SESSION_BLOCK_DIRECTIONS
 #endif
 
 extern const struct Curl_handler Curl_handler_scp;