[GRASS-git] [OSGeo/grass] fedef5: Fix -Wimplicit-fallthrough warnings (#2760)

[Nicklas Larsson]

  Branch: refs/heads/main
  Home:   https://github.com/OSGeo/grass
  Commit: fedef50a866e47846d46cf475b795719407ed8f3
      https://github.com/OSGeo/grass/commit/fedef50a866e47846d46cf475b795719407ed8f3
  Author: Nicklas Larsson <n_larsson at yahoo.com>
  Date:   2023-02-14 (Tue, 14 Feb 2023)

  Changed paths:
    M .github/workflows/gcc.yml
    M display/d.labels/do_labels.c
    M imagery/i.atcorr/geomcond.cpp
    M include/grass/gis.h
    M lib/bitmap/bitmap.c
    M lib/gmath/la.c
    M lib/nviz/map_obj.c
    M lib/ogsf/gvl_calc2.c
    M ps/ps.map/scan_ref.c
    M vector/v.in.dxf/dxf_to_vect.c

  Log Message:
  -----------
  Fix -Wimplicit-fallthrough warnings (#2760)

* define FALLTHROUGH attribute statement macro and use it where needed
* break switch cases where needed
* CI: remove exempted compiler warning