[R-SIG-Mac] Compling rgl for Mavericks

[Duncan Murdoch]

The rgl package compiles and runs on Mavericks, though it only displays 
in X11 currently, and it gets a number of compile-time warnings like

glgui.cpp:50:5: warning: 'gluUnProject' is deprecated: first deprecated 
in OS X 10.9
       - "Use GLKMathUnproject" [-Wdeprecated-declarations]

There are also warnings about gluProject, gluErrorString, and some others.

I can probably write conditional workarounds for each of these, but I 
have a very basic question:  how do I write a conditional to depend on 
an OS X version?  Do I need to write my own autoconf macro, or is there 
one available in the standard collection, or online somewhere?

Duncan Murdoch