C compilation fixes and hardening
    
    Fix "implicit declaration of function ‘strlen’" in ipa_pwd_ntlm.c,
    credits to Lukas.
    
    Add -Werror=implicit-function-declaration to CFLAGS to point developers
    to missing includes. It causes compilation to fail when a developer
    forgets to add a required include. The problem is no longer hidden in a
    massive wall of text from make.
    
    Silence a harmless error from 389-DS slapi.h until the bug is fixed in
    downstream, https://pagure.io/389-ds-base/issue/48979
    
    Signed-off-by: Christian Heimes <cheimes@redhat.com>
    Reviewed-By: Stanislav Laznicka <slaznick@redhat.com>