deskutils/gnome-clocks: fix build on GCC architectures

Use C11 compiler:
/usr/local/include/libhandy-1/hdy-swipe-tracker.h:23: error: redefinition of typedef 'HdySwipeTracker'
/usr/local/include/libhandy-1/hdy-types.h:15: error: previous declaration of 'HdySwipeTracker' was here