glXSwapIntervalEXT() does not return a value, contrary to the
SGI equivalent. In case of error, the X11 error handler is used.

This patch fixes compilation with conforming headers. It should still
be compatible with buggy Mesa headers where 'int' is returned.

For reference, see the specification here:
http://www.opengl.org/registry/specs/EXT/swap_control.txt
http://www.opengl.org/registry/api/glxext.h