WIP: define _XOPEN_SOURCE to 700 for strptime(3)
Defining _OPEN_SOURCE alone is not enough, it must be set to 700 to identify POSIX 2008 on at least OpenBSD to prevent the following warning:
../gnome-todo-3.28.1/plugins/todoist/gtd-provider-todoist.c:293:8: warning: implicit declaration of function 'strptime' is invalid in C99 [-Wimplicit-function-declaration] if (!strptime (due_date, "%a %d %b %Y %T %z", &due_dt))