Code:
/usr/ports/devel/llvm19/work-default/llvm-project-19.1.7.src/llvm/tools/gold/gold-plugin.cpp:40:10: fatal error: 'plugin-api.h' file not found
40 | #include <plugin-api.h>
Turns out that plugin-api.h is part of the binutils package which the llvm19 build did not insist on as a dependency for some reason. Once installed, no more missing file.