Sun Solaris 9 sparc. Xos_r.h ?

Hi,

I want to learn Motif, and downloaded the source tarball. After some messing with $PATH on my machine, I was able to create the makefile. Unfortunately, my joy was short lived. The system is looking for a library called Xos_r.h, and a search does not locate it anywhere on my drives. Anyone know where I can obtain it?

TIA,

Colin.