What distribution of Debian are you using (stable, testing, ...)?
In /usr/share/doc/libstdc++6-4.3-dev/C++/changelog.libstdc++.gz, I see
2009-01-08 Paolo Carlini <address@hidden>
Backport from mainline:
2008-11-13 Paolo Carlini <address@hidden>
PR libstdc++/38000
* include/c_global/csignal: Do not use include_next.
...
* include/c_global/cstring: Likewise.
...
The bug report about this problem is here:
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=38000
So I think you should install and use a fixed version of GCC rather
than modify Octave to work around this problem.
jwe