hygiene, lose anon-init case in date-core.h auxiliaries

Signed-off-by: Sebastian Freundt <freundt@ga-group.nl>
hygiene, use sloppy initted compound literal if compiler supports it

This change will turn a superfluous:

   memset(&x, 0, sizeof(x));
   return x;

into a leaner:

   return (x){};

But gcc decides to warn about missing initialisers so
the leaner code is conditionalised on HAVE_SLOPPY_INIT_STRUCT.

Signed-off-by: Sebastian Freundt <freundt@ga-group.nl>
Merge branch 'code/hygiene'

* code/hygiene:
  hygiene, use sloppy initted compound literal if compiler supports it
  hygiene, lose anon-init case in date-core.h auxiliaries