add comments that AC_USE_SYSTEM_EXTENSIONS and AST_PROG_LD

add comments that AC_USE_SYSTEM_EXTENSIONS and AST_PROG_LD
do not work on FreeBSD - presumably they depend on some
auto* feature that is not installed by default.

I am not sure on what is a proper fix. In my local copy
i simply comment them out.

The AST_PROG_LD is a long standing isse, there were attempts
to fix it in the past but probably not enough has been copied
to acinclude.m4, and i had forgotten about it because i
commented out this call in configure.ac long ago

git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@51288 65c4cc65-6c06-0410-ace0-fbb531ad65f3