[InterNetNews/inn] a46e50: Add new gcc warnings flags

[Julien ÉLIE]

  Branch: refs/heads/main
  Home:   https://github.com/InterNetNews/inn
  Commit: a46e50d67325712f551bf261ea065f8f827bed08
      https://github.com/InterNetNews/inn/commit/a46e50d67325712f551bf261ea065f8f827bed08
  Author: Julien ÉLIE <Julien-Elie at users.noreply.github.com>
  Date:   2021-11-22 (Mon, 22 Nov 2021)

  Changed paths:
    M configure.ac

  Log Message:
  -----------
  Add new gcc warnings flags

Check against gcc 10.2.1.

Enable -D_FORTIFY_SOURCE=2 now that there are no warnings for unused
results (fixed in previous commits).

Enable -Warith-conversion -Warray-bounds=2 -Wattribute-alias=2

Mark -Wswitch-enum as producing useless noise.