$ diff -c emacs-23.4/src/xgselect.c xgselect-patch.c > xgselect_c.diff

$ patch --verbose < xgselect_c.diff 
Hmm...  Looks like a new-style context diff to me...
The text leading up to this was:
--------------------------
|*** emacs-23.4/src/xgselect.c	2012-01-11 13:35:01.000000000 +0100
|--- xgselect-patch.c	2012-03-03 13:45:43.737000008 +0100
--------------------------
Patching file xgselect-patch.c using Plan A...
Reversed (or previously applied) patch detected!  Assume -R? [n] 
Apply anyway? [n] y
Hunk #1 FAILED at 55.
Hunk #2 FAILED at 152.
2 out of 2 hunks FAILED -- saving rejects to file xgselect-patch.c.rej
done