Not so fast. Adding stddef.h did nothing. Slackware64-current places a kernel-installed stddef in /usr/include/stddef.h that defines inline. Gcc v9 places stddef in /usr/lib64/gcc/x86_64-slackware-linux/9.2.0/include/stddef.h. Adding that include path to the gcc command doesn't work. I had to add to adl_structures.h this include in order to compile adlutil:
#include </usr/lib64/gcc/x86_64-slackware-linux/9.2.0/include/stddef.h>
also
#include <stdbool.h>
to clear another compile error. Of course, once I had an executable adlutil, I couldn't use it because Libatiadlxx.so does not exist. The display library from git does not include any libraries, contrary to its name. Sew! I downloaded the Open64 SDK, which consists of files about ten years old. I can find nothing to support my relatively new 17" laptop Ryzen 5 2500U with Radeon Vega mobility gfx.