Log Message:
Update to 2.64.
This also includes the fix for PR pkg/26386 (problems with
PKG_CONFIG=no).

Summary of major changes since 2.63
-----------------------------------

  - Security fix prevents a denial of service attack open to certain
    malformed messages; this DoS affects all SpamAssassin 2.5x
    and 2.6x versions to date.
  - Backported several very reliable rules from the SpamAssassin 3.0.0
    codebase.