Add zsync package

zsync is a client-side file transfer program similar to rsync.

Tested as working on Samsung Chromebook Plus (aarch64). All tests
passing.

packages/zsync.rb

+require 'package'
+
+class Zsync < Package
+  description 'zsync is a client-side file transfer program similar to rsync.'
+  homepage 'http://zsync.moria.org.uk/'
+  version '0.6.2'
+  source_url 'https://zsync.moria.org.uk/download/zsync-0.6.2.tar.bz2'
+  source_sha256 '0b9d53433387aa4f04634a6c63a5efa8203070f2298af72a705f9be3dda65af2'
+
+  def self.preinstall
+    system "curl -Ls -o autotools/config.guess 'https://git.savannah.gnu.org/gitweb/?p=config.git;a=blob_plain;f=config.guess;hb=HEAD'"
+    system "curl -Ls -o autotools/config.sub 'https://git.savannah.gnu.org/gitweb/?p=config.git;a=blob_plain;f=config.sub;hb=HEAD'"
+  end
+
+  def self.build
+    system './configure',
+           "--prefix=#{CREW_PREFIX}",
+           "--libdir=#{CREW_LIB_PREFIX}",
+           '--disable-maintainer-mode'
+    system 'make'
+  end
+
+  def self.install
+    system 'make', "DESTDIR=#{CREW_DEST_DIR}", 'install'
+  end
+end