Some compilers seem to be over-pessimistic and abort the build with:
gcal-month-view.c: In function 'gcal_month_view_drag_drop':
gcal-month-view.c:908:32: error: 'timespan' may be used uninitialized in this function [-Werror=maybe-uninitialized]
GDateTime *new_end = g_date_time_add (new_start, timespan);
Even though the usage of the variable is guarded by an equal if as the assignment
https://bugzilla.gnome.org/show_bug.cgi?id=770667
