Andrea,
You can get past the "CMake Error at CMakeLists.txt:65 (pkg_check_modules): Unknown CMake command "pkg_check_modules"."
by adding this line in cinelerra-5.1/CMakeLists.txt
find_package(PkgConfig REQUIRED)
before line 65 which reads:
pkg_check_modules(X11 REQUIRED IMPORTED_TARGET x11)
I will make note of this in the Bug Tracker qt6 category now that I know it is just not my problem.
However, I next get this error which I have not diagnosed yet:
make[2]: *** [cinelerra/CMakeFiles/cin.dir/build.make:7479: cinelerra/CMakeFiles/cin.dir/transcribe_engine.C.o] Error 1
Please let me know if you get this same error further along on your next attempt so I know it is just not me. Thank you, Phyllis