Use C11 compiler:
In file included from /usr/local/include/xfce4/libxfce4util/libxfce4util.h:33,
from calculator.c:29:
/usr/local/include/xfce4/libxfce4util/xfce-kiosk.h:37: error: redefinition of typedef 'XfceKiosk'
/usr/local/include/xfce4/libxfce4util/xfce-kiosk.h:35: error: previous declaration of 'XfceKiosk' was heremv -f .deps/libcalculator_la-lexer.Tpo .deps/libcalculator_la-lexer.Plo
Approved by: mentors (implicit approval)