include XZ as dependency for libunwind

[boegel]

run into this while testing #2892

workaround with LIBS is easier than patching the broken Makefile, cfr. https://bugs.gentoo.org/show_bug.cgi?id=444050

[boegel]

Test report by @boegel
SUCCESS
Build succeeded for 9 out of 9 (9 easyconfigs in this PR)
node2014.delcatty.os - Linux SL 6.7, Intel(R) Xeon(R) CPU E5-2670 0 @ 2.60GHz, Python 2.6.6
See https://gist.github.com/e49b7ad70e37ddfa97abbbf1ad67e006 for a full test report.

[boegel]

Test report by @boegel
SUCCESS
Build succeeded for 1 out of 1 (1 easyconfigs in this PR)
node2573.golett.os - Linux centos linux 7.2.1511, Intel(R) Xeon(R) CPU E5-2680 v3 @ 2.50GHz, Python 2.7.5
See https://gist.github.com/49e83bfa5862138cddd69d721fa6b46a for a full test report.