Maintenance: add .xz tarball format formally to make dist (squid-cach…

…e#325)

Automake can now handle generating this format itself and the
experiments of providing it for downstream have gone well.

configure.ac

@@ -10,7 +10,7 @@ AC_PREREQ(2.61)
 AC_CONFIG_HEADERS([include/autoconf.h])
 AC_CONFIG_AUX_DIR(cfgaux)
 AC_CONFIG_SRCDIR([src/main.cc])
-AM_INIT_AUTOMAKE([tar-ustar nostdinc subdir-objects])
+AM_INIT_AUTOMAKE([tar-ustar nostdinc subdir-objects dist-xz])
 AC_REVISION($Revision$)dnl
 AC_PREFIX_DEFAULT(/usr/local/squid)
 AM_MAINTAINER_MODE