Don't compile WriteMonoDepthSpan if HAVE_HW_DEPTH_SPANS is defined -- it won't

be used in that case, and it wanting WRITE_DEPTH was making r128 die on
the undefined symbol.

src/mesa/drivers/dri/common/depthtmp.h

@@ -64,6 +64,7 @@ static void TAG(WriteDepthSpan)( GLcontext *ctx,
    HW_WRITE_UNLOCK();
 }
 
+#if !HAVE_HW_DEPTH_SPANS
 static void TAG(WriteMonoDepthSpan)( GLcontext *ctx,
                                  GLuint n, GLint x, GLint y,
 				 const GLdepth depth,
@@ -99,6 +100,7 @@ static void TAG(WriteMonoDepthSpan)( GLcontext *ctx,
       }
    HW_WRITE_UNLOCK();
 }
+#endif
 
 static void TAG(WriteDepthPixels)( GLcontext *ctx,
 				   GLuint n,