As of 24 December I was getting an error with dev-util/gnome-builder-3.38.1 and dev-util/sysprof-3.38.1 with the sysprof flag active. My compile config:

dev-util/gnome-builder-3.38.1 clang devhelp git glade gtk-doc spell sysprof  PYTHON_SINGLE_TARGET="python3_8"
dev-util/sysprof-3.38.1 gtk unwind

Sysprof has a /usr/include/sysprof-3 with duplicate headers that get picked up first and give redefine error when the same headers are included from sysprof-4. My quick and dirty fix:

cd /usr/include/sysprof-3

mv sysprof-version-macros.h _sysprof-version-macros.h
mv sysprof-clock.h _sysprof-clock.h
mv sysprof-capture-types.h _sysprof-capture-types.h
mv sysprof-capture-reader.h _sysprof-capture-reader.h
mv sysprof-address.h _sysprof-address.h
mv sysprof-capture-condition.h _sysprof-capture-condition.h
mv sysprof-capture-writer.h _sysprof-capture-writer.h
mv sysprof-collector.h _sysprof-collector.h
mv sysprof-capture-cursor.h _sysprof-capture-cursor.h
mv sysprof-platform.h _sysprof-platform.h
mv sysprof-capture.h _sysprof-capture.h