I just tried to compile the latest AzerothCore on Debian 9 and did not touch the source, just pulled it and used exactly the sh described in the installation guide. CMake gives me that error:
CMake Error: The following variables are used in this project, but they are set to NOTFOUND.
Please set them or make sure they are set and tested correctly in the CMake files:
ACE_INCLUDE_DIR (ADVANCED)
Debian 9 with the latest AzerothCore. Ace is definitely installed with their latest version.
Expected: No error. Actual Result: Error.