handle tar.xz tarballs

Change-Id: I8e36ad989ec54117f85105c24bc1c1442e0a493b
Reviewed-on: https://gerrit.libreoffice.org/15454Reviewed-by: Andras Timar <andras.timar@collabora.com>
Tested-by: Andras Timar <andras.timar@collabora.com>

solenv/gbuild/UnpackedTarball.mk

@@ -51,6 +51,9 @@ $(dir $(call gb_UnpackedTarget_get_target,%)).dir :
 $(call gb_UnpackedTarget_get_target,%).tar.bz2 :
 	$(call gb_UnpackedTarget__command,untar,$@,$*,-j)
 
+$(call gb_UnpackedTarget_get_target,%).tar.xz :
+	$(call gb_UnpackedTarget__command,untar,$@,$*,-J)
+
 $(call gb_UnpackedTarget_get_target,%).tar.gz :
 	$(call gb_UnpackedTarget__command,untar,$@,$*,-z)