From build/README:

- Rule Source is only in trunk.  If you are building a branch, checkout
  trunk as well and symlink it, i.e. rulesrc -> ../trunk/rulesrc/

- t.rule Source is only in trunk.  If you are building a branch, checkout
  trunk as well and symlink it, i.e. t.rules -> ../trunk/t.rules/

- Rules are ONLY published from trunk.  Rule development should use plugin
  and version conditions to make it so one ruleset works on all modern
  versions of SA. If you are building a branch, checkout trunk as well and
  symlink the rules from trunk, i.e. rules -> ../trunk/rules/

  With the rules in trunk symlinked, you can expect MANIFEST warnings when
  running things such as make distclean such as:

    No such file: rules/20_aux_tlds.cf
    No such file: rules/active.list
    No such file: rules/init.pre
    No such file: rules/languages
    No such file: rules/local.cf
    No such file: rules/regression_tests.cf
    No such file: rules/sa-update-pubkey.txt
    No such file: rules/user_prefs.template
    No such file: rules/v310.pre
    No such file: rules/v312.pre
    No such file: rules/v320.pre
    No such file: rules/v330.pre
    No such file: rules/v340.pre
    No such file: rules/v341.pre
    No such file: rules/v342.pre

  NOTE: Don't remove the lines from the MANIFEST though!