Building Octave without xcode

[Jordi Gutiérrez Hermoso]

Is it impossible to build Octave without Xcode? Ben just pushed a
patch (11410:2df163be223e) for README.MacOS which suggests installing
Xcode in order to build Octave. This breaks with the GNU coding
standards:

     http://www.gnu.org/prep/standards/html_node/References.html

There's nothing wrong with helping people install Octave on Mac OS X,
but we should be able to recommend a method that doesn't depend on
installing non-free software beyond the core OS and libraries.

- Jordi G. H.