Handling C++11

[Ken Cunningham]

Octave builds and runs great on older systems right now when they are
upgraded to libc++ per the current instructions.

$ port -v installed octave
The following ports are currently installed:
  octave @4.2.0_1+accelerate+app+docs+fltk+gfortran+graphicsmagick+qt4+sound
platform='darwin 10' archs='x86_64'
  octave @4.2.0_2+accelerate+app+docs+fltk+gfortran+graphicsmagick+qt4+sound
(active) platform='darwin 10' archs='x86_64'


Rather than force the older systems to install gcc6 and libgcc and
then find a roundabout and complicated way to support cxx11 with c++
standard library inconsistencies, I would suggest we finally just
change/add/modify/get the buildbots to libc++ once and for all and be
done with that.

IMHO it would be a lot less trouble to do that than it will be to go
through the trouble of working through all the ramifications of yet
another standard library on these systems.