Autodetection of libarchive version installed by Brew

[Changaco]

The version of libarchive included in Mac OS X is obsolete, consequently many people use Homebrew to install another version, but then they have to set the LIBARCHIVE environment variable, otherwise the obsolete version is still used by the Python module.

It could be a good idea to automatically detect and use by default the libarchive version installed by Brew, by doing something similar to the shell one-liner find "$(brew --cellar libarchive)" -name libarchive.13.dylib | sort | tail -1 and the Python script brew_find_libarchive.