make --with-gssapi work automatically in FreeBSD

This is a copy-paste from QA jenkinsfiles showing a difference in compilation flags:

if (system == 'freebsd') {
  cfgOpts += ' --with-gssapi=/usr/local/bin/krb5-config'
} else {
  cfgOpts += ' --with-gssapi'
}

It would be nice if --with-gssapi would also look in /usr/local/bin to find krb5-config automatically in FreeBSD.