binary-dist: when using xz, use extreme compression.

When building a binary distribution with TAR_COMP=xz, using the -9e flag
(extremely high compression) results in substantial savings: for the
Mavericks builds, bzip2 scores in at about 120mb, while xz at level 9
scores about 60mb - a huge reduction!

This of course takes significantly longer - but it does not affect
decompression speed for end users, so it's certainly worth it.

Signed-off-by: Austin Seipp <austin@well-typed.com>