You're right—the PATH was missing. After adding it to /usr/include, the compilation started. It stopped after 1–2 minutes with the following message:

1052:/home/paz/cinelerra-gg-qt6/cinelerra-5.1/guicast/errorbox.h:35:14: note:   by ‘void ErrorBox::create_objects(const char*)’


Looking at .../cinelerra-5.1/guicast/errorbox.h on line 35, I find the following line:

void create_objects(const char *text);


But I don't know how to proceed from here. I'm attaching the build.log file.
https://paste.c-net.org/TurksKeith