heimdal build clang 20: compile lex generated code

heimdal build clang 20: compile lex generated code

The lex/flex generated code in the heimdal build triggers:
   strict-prototype and implicit-fallthrough errors

Rather that blanket disabling the warnings, try to disable them only
when compiling lex/flex generated code

Signed-off-by: Gary Lockyer <gary@catalyst.net.nz>
Reviewed-by: Douglas Bagnall <douglas.bagnall@catalyst.net.nz>
Reviewed-by: Anoop C S <anoopcs@samba.org>
Autobuild-User(master): Douglas Bagnall <dbagnall@samba.org>
Autobuild-Date(master): Thu May 29 00:19:50 UTC 2025 on atb-devel-224