See here:

http://bugzilla.icculus.org/show_bug.cgi?id=4064

code/renderer/tr_local.h

@@ -1008,6 +1008,7 @@ extern cvar_t	*r_stencilbits;			// number of desired stencil bits
 extern cvar_t	*r_depthbits;			// number of desired depth bits
 extern cvar_t	*r_colorbits;			// number of desired color bits, only relevant for fullscreen
 extern cvar_t	*r_texturebits;			// number of desired texture bits
+extern cvar_t	*r_ext_multisample;
 										// 0 = use framebuffer depth
 										// 16 = use 16-bit textures
 										// 32 = use 32-bit textures