No extern "C" for XFree86 OpenGL headers

[Tron Thomas]

The XFree86 versions of the OpenGL headers files do not use extern "C" 
when compiling C++ modules.  This can result in linking errors as OpenGL 
function receive decorated signatures that won't exist in the libraries.

A work around is for a developer to do something like the following in a 
C++ code module:
#ifdef __cplusplus
extern "C" {
#include <GL/gl.h>
}
#endif

which shouldn't normally be necessary as other implementations of OpenGL 
do not require this.