Makevars that distinguish between OSX and *NIX

Hi

Is there a similar way to create a specific Makevars for OSX versus *NIX
systems as for Windows, e.g. a Makervars.osx ?
If not what is there another lean solution to create system specific
Makevars? I need to link differently depending on whether I am on *NIX
or OSX (specifically link to OpenGL).

Well, OS X (sic) is a certified UNIX!

This is what a 'configure' script is used for (e.g. by package rgl). 
But you can write conditional code in commands in the Makevars, e.g. by

if test `uname` = "Darwin" ; then
else
fi

Please do not use GNU make misfeatures to do this if you want it to be 
portable (see 'Writing R Extensions').

Brian D. Ripley,                  ripley at stats.ox.ac.uk
Professor of Applied Statistics,  http://www.stats.ox.ac.uk/~ripley/
University of Oxford,             Tel:  +44 1865 272861 (self)
1 South Parks Road,                     +44 1865 272866 (PA)
Oxford OX1 3TG, UK                Fax:  +44 1865 272595