possible libtools bug with AC_PREFIX_DEFAULT

[Matthew Chan]

Hi Everyone, 

While attempting to package brlcad, I came across a potential bug
involving libtools and the AC_PREFIX_DEFAULT env variable. One of the
more knowledgeable users in ##workingset suggested that I report it
here.

When installing with 'make DESTDIR=... install', I would get the "cannot
install x to a directory not ending in y" error. 

The pastebin of the output is here: http://fpaste.org/paste/20546

However, builds using --prefix and not DESTDIR would complete
successfully.

We looked over the Makefile.[am,in] and also the configure.am, and
--libdir wasn't being used anywhere. The only thing that we could think
of was the AC_PREFIX_DEFAULT env variable being configured
to /usr/brlcad/

The configure.ac is here: http://fpaste.org/paste/20561 
The Makefile.am is pastebinned here: http://fpaste.org/paste/20558
The Makefile.in is here: http://fpaste.org/paste/20562 

The libtool version is 2.2.6.

Is this a bug on the part of libtools, or is it improper use?

Thanks,
Matt