Use xz compression as it saves space compared to gz.

Signed-off-by: Daniel Engberg <daniel.engberg.lists@pyret.net>

From the upstream NEWS file:

  r131
  New    : Dos/DJGPP target, thanks to Louis Santillan (#114)
  Added  : Example using lz4frame library, by Zbigniew Jędrzejewski-Szmek (#118)
  Changed: xxhash symbols are modified (namespace emulation) within liblz4

  r130:
  Fixed  : incompatibility sparse mode vs console, reported by Yongwoon Cho (#105)
  Fixed  : LZ4IO exits too early when frame crc not present, reported by Yongwoon Cho (#106)
  Fixed  : incompatibility sparse mode vs append mode, reported by Takayuki Matsuoka (#110)
  Performance fix : big compression speed boost for clang (+30%)
  New    : cross-version test, by Takayuki Matsuoka

Signed-off-by: Darik Horn <dajhorn@vanadac.com>

Signed-off-by: Darik Horn <dajhorn@vanadac.com>