Debug information file directory variable?

[Jeffrey Walton]

I checked my copy of Stallman's GNU Make and the online guide ("15.5
Variables for Installation Directories" at
http://www.gnu.org/software/make/manual/html_node/Directory-Variables.html),
and I'm having trouble answering this question.

The "debug information file" is what we get when we do something like
the following. Its the file with the ".debug" extension.

    objcopy --only-keep-debug libbar.so libbar.so.debug
    strip --strip-debug --strip-unneeded libbar.so
    objcopy --add-gnu-debuglink=$(XXX)/libbar.so.debug libbar.so

We have 'prefix', 'exec_prefix', and friends, but I don't see
something for debug. In a makefile, we will need an install rule that
places libbar.so.debug where it belongs on the filesystem.

What are the GNU Make directory variable(s) for the debug information
file? Or how should it be formed?

Related, and more generally: does GNU have a policy governing or
directing the handling of the debug information file? The Binutils and
GDB documentation is sending mixed signals and providing contradictory
information. And platforms based on Linux and GNU seem to be
different, too.

Thanks in advance.