libarchive now default

[Gabriel M. Beddingfield]

If you are building from Git, Composite now requires
libarchive-dev as the default instead of libtar and libz.
If you don't already have it installed, it's recommended.

The switch is made because libtar is apparently not under
active development, and is designed in a way that doesn't
work well in other shared libraries.[1] This is known to
create a build error on some 64-bit Linux distros.
Meanwhile, libarchive is the default choice for Windows and
OS X... so it makes sense to use the same for all 3.

Peace,
Gabriel

[1] http://bugs.archlinux.org/task/13929