heimdal_build: Improve error and warning handling on old and new compilers

The previous commit 1eadeaed0a6ca3a58eb9fd176a7ae5bcc28f64ef had a couple of
errors, the unpicky flags were being set on all builds (not just old
compiler builds) due to confusing variable names, and Ubuntu 16.04
would not build (for fuzzing) because it thought some variables
were maybe-uninitialized.

This keeps stricter warnings->errors on modern compilers while
allowing the full build, even in the near future when a modern
Heimdal is imported.

Signed-off-by: Andrew Bartlett <abartlet@samba.org>
Reviewed-by: Stefan Metzmacher <metze@samba.org>

Autobuild-User(master): Stefan Metzmacher <metze@samba.org>
Autobuild-Date(master): Wed Jun 16 14:43:17 UTC 2021 on sn-devel-184
