Post-release fixes are kept here. 

To use a source code patch you must have a full unmodified version of the
XFree86 4.2.1 source tree.  These patches cannot be applied to a binary
distribution.  To apply a patch, run the following from the directory
containing the 'xc' directory:

  patch -p0 < patch-file

A full rebuild ("make World") is recommended after applying patches.