visibility attributes with GCC

[Rik]

1/3/15

jwe,

This looks like it should be reasonably easy to implement since all of the
code is already marked up with OCTAVE_API.  But getting everything right
will probably be a project since there are a lot of loose ends to track
down.  This page had good basic information about the desirability of using
visibility attributes and a step-by-step approach to implementing it in a
project (https://gcc.gnu.org/wiki/Visibility).

--Rik