Replace glu with GLKit on OSX

[tpaviot]

Since OSX 10.9 is released, clang raises some deprecated warnings related to the use of glu, for instance:

/oce/src/OpenGl/OpenGl_GraduatedTrihedron.cxx:60:3: warning: 'gluUnProject' is deprecated: first deprecated in OS X 10.9 
"Use GLKMathUnproject" [-Wdeprecated-declarations]
  gluUnProject(viewport[0], viewport[1], 0., model_matrix, proj_matrix, viewport, &x1, &y1, &z1);
  ^

Instead of glu, Apple suggests to move to the GLKit library (https://developer.apple.com/library/ios/documentation/GLkit/Reference/GLKit_Collection/_index.html). I think it is for portability reasons with iOS.

This issue is not critical: we can only disable the warning, but we should keep it in mind.