Merge pull request #3835 from qlyoung/build-update-libyang-doc-link

build: update doc link for libyang warning

diff --cc configure.ac
index 488a1380b2a5877ef4472049beb8e7fecfce8dbd,bf86b69ceb05dc3d089d3f7ef519a7dfedcd85e4..157dad33a91290b591e7ffc1e77d777940a4c9ba
--- 1/configure.ac
--- 2/configure.ac
+++ b/configure.ac
@@@ -1611,28 -1598,9 +1611,28 @@@ CFLAGS="$CFLAGS $LIBYANG_CFLAGS
  AC_CHECK_MEMBER([struct lyd_node.priv], [], [
    AC_MSG_ERROR([m4_normalize([
      libyang needs to be compiled with ENABLE_LYD_PRIV=ON.
-     See http://docs.frrouting.org/projects/dev-guide/en/latest/building-libyang.html for details.])
+     Instructions for this are included in the build documentation for your platform at http://docs.frrouting.org/projects/dev-guide/en/latest/building.html])
    ])
  ], [[#include <libyang/libyang.h>]])
 +CFLAGS="$ac_cflags_save"
 +
 +ac_libs_save="$LIBS"
 +LIBS="$LIBS $LIBYANG_LIBS"
 +AC_CHECK_FUNC([ly_register_types], [
 +  libyang_ext_builtin=true
 +  AC_DEFINE([LIBYANG_EXT_BUILTIN], [1], [have ly_register_types()])
 +], [
 +  libyang_ext_builtin=false
 +  AC_MSG_WARN([===== old libyang (before 0.16.74) detected =====])
 +  AC_MSG_WARN([The available version of libyang does not seem to support])
 +  AC_MSG_WARN([built-in YANG extension modules.  This will cause "make check"])
 +  AC_MSG_WARN([to fail and may create installation and version mismatch issues.])
 +  AC_MSG_WARN([Support for the old mechanism will be removed at some point.])
 +  AC_MSG_WARN([Please update libyang to version 0.16.74 or newer.])
 +  AC_MSG_WARN([===== old libyang (before 0.16.74) detected =====])
 +])
 +AM_CONDITIONAL([LIBYANG_EXT_BUILTIN], [$libyang_ext_builtin])
 +LIBS="$ac_libs_save"
  
  dnl ---------------
  dnl configuration rollbacks