[Apologies if this is not news; I searched the mailing list archive, but didn't find anything about this.]
I have just been trying to debug some libtoolized code on macOS 10.12 (Sierra).
I am using lldb, to avoid going through some signing rigmarole with gdb.
When I start it up it complains about not being able to find a dynamic library in /usr/local/lib, which is correct, as it's not there. On investigation, DYLD_LIBRARY_PATH is correctly set, but is ignored.
As far as I can tell, this is by design: the System Integrity Protection feature of macOS 10.11 and later disables DYLD_LIBRARY_PATH.
This means that dynamic libraries can't be loaded in-place using current libtool (I am using 2.4.6).
At the moment, my workaround will be to install the library in /usr/local/lib, as I don't want to disable SIP on the Mac I'm borrowing to test a Darwin build of my code!
I guess it won't be long before such security mechanisms surface in popular GNU/Linux distros…
I presume that the measures that XCode uses to get around this could be used in GNU libtool (though I confess I don't know what they are, and right now don't plan to look, as I'm not a regular Darwin user, just someone trying to make code compile on this popular platform, sigh).