Making all in cups...
[…]
Compiling tls.c...
In file included from tls.c:39:0:
tls-gnutls.c: In function ‘httpCredentialsAreValidForName’:
tls-gnutls.c:420:56: error: conversion to ‘int’ from ‘unsigned int’ may change the sign of the result [-Werror=sign-conversion]
           if (!gnutls_x509_crl_get_crt_serial(tls_crl, (unsigned)i, rserial, &r
                                                        ^
cc1: all warnings being treated as errors