The failing build step output is: FAILED: libgnucash/app-utils/swig-app-utils-python.c libgnucash/app-utils/sw_app_utils.py /var/tmp/portage/app-office/gnucash-4.13/work/gnucash-4.13_build/libgnucash/app-utils/swig-app-utils-python.c /var/tmp/portage/app-office/gnucash-4.13/work/gnucash-4.13_build/libgnucash/app-utils/sw_app_utils.py cd /var/tmp/portage/app-office/gnucash-4.13/work/gnucash-4.13_build/libgnucash/app-utils && /usr/bin/swig -python -py3 -Wall -Werror -I/usr/include/glib-2.0 -I/usr/lib64/glib-2.0/include -I/usr/include -I/var/tmp/portage/app-office/gnucash-4.13/work/gnucash-4.13/common -I/var/tmp/portage/app-office/gnucash-4.13/work/gnucash-4.13/libgnucash/engine -I/var/tmp/portage/app-office/gnucash-4.13/work/gnucash-4.13/libgnucash/app-utils -I/var/tmp/portage/app-office/gnucash-4.13/work/gnucash-4.13/bindings -I/var/tmp/portage/app-office/gnucash-4.13/work/gnucash-4.13/bindings/python -o /var/tmp/portage/app-office/gnucash-4.13/work/gnucash-4.13_build/libgnucash/app-utils/swig-app-utils-python.c /var/tmp/portage/app-office/gnucash-4.13/work/gnucash-4.13/libgnucash/app-utils/app-utils.i Deprecated command line option: -py3. Ignored, this option is no longer supported. /usr/include/guid.h:41: Warning 301: class keyword used, but not in C++ mode. /usr/include/guid.h:83: Warning 301: class keyword used, but not in C++ mode. ninja: build stopped: subcommand failed. This is due to the build picking up /usr/include/guid.h installed by dev-libs/crossguid instead of libgnucash/engine/guid.h in the gnucash source tree. I believe the include is coming from bindings/python/gnucash_core.i. Temporarily renaming /usr/include/guid.h allows the gnucash build to succeed.