Template code to use lt_dladvise()

Template code to use lt_dladvise()

The libtool people have discovered that it's useful to allow
*additional* features from linking.  Like allowing libraries to
link to libraries.  Using this magic API allows FreeRADIUS to load
the Perl module, which in turn loads other modules, which in turn
load dynamic libraries.  Right now, it complains because of
limitations in libltdl.

We COULD do these checks in 2.1.8, but that involves either adding
configure checks, OR upgrading our version of libltdl.  We don't want
to do either.