xFoo moved to octave namespace -- avoid warnings about deprecated functions for Forge package

[Carnë Draug]

You may have noticed that Octave has been moving a lot of stuff into
the octave namespace while deprecating the versions on the main
namespace.  For example, xlog2, xround, xisnan, xmin, and xmax.

If a package with oct files wants to support both the upcoming Octave
release and current previous Octave releases, this will trigger deprecation
warnings, which may cause a lot of new bug reports.

I have solved this for the image package by checking at configure time,
creating a wrapper that uses what's not deprecated, and use that wrapper
instead [1, 2]. May be of interest to other package maintainers.

Carnë

[1] http://hg.code.sf.net/p/octave/image/rev/5edfaa3cce35
[2] http://hg.code.sf.net/p/octave/image/rev/672b4b8dd371