Added some packetver-related flags to the UNIX build script

- The --disable-packetver-re (or --enable-packetver-re=no) flag
  prevents the definition of PACKETVER_RE (without editing mmo.h)
- The --with-key1=, --with-key2=, --with-key3= flags override the
  encryption key defined by the current packetver. All three flags are
  required if at least one is used, or they'll be ignored.
- These options are mostly useful for buildbots, developers who often
  use git bisect, or users who want to minimize their diffs and still
  want to override those settings.
- (unrelated minor tweak) Silenced an unnecessarily verbose STDERR
  message caused by the $CC shipped with clang-5 during the MinGW check.