These are header files for preliminary MIT-SHM support, which belong into
\emx\include\sys. \XFre86\lib\shm.{a,lib} are the forwarder libs for the 
DLL at \XFree86\lib\shm.dll

It is unclear at this moment whether the provided shm.dll actually works, 
so be prepared to crash the server when trying to use MIT-SHM. It is 
expectable though that these header files and libs will be replaced by 
working code in order to port X apps that rely on the SHM extension, and
that EMX will maybe have this support already included.

