Add a special #define for OCIO in MacOS

OCIO doesn't support GLSL 1.5, and MacOS's #version 150 core
doesn't support calls to texture3d.

Source: https://lists.blender.org/pipermail/bf-blender-cvs/2017-August/100014.html

Differential Revision: https://phabricator.kde.org/D13485