configure: Use ustar format for dist tarball

Since for stable releases, some test files were over the 99 char
limit for traditional tar filenames.

Suggested by Osier here:
https://www.redhat.com/archives/libvir-list/2012-April/msg01435.htmlSigned-off-by: NCole Robinson <crobinso@redhat.com>

configure.ac

@@ -9,7 +9,7 @@ AC_CONFIG_AUX_DIR([build-aux])
 AC_CONFIG_HEADERS([config.h])
 AC_CONFIG_MACRO_DIR([m4])
 dnl Make automake keep quiet about wildcards & other GNUmake-isms
-AM_INIT_AUTOMAKE([-Wno-portability])
+AM_INIT_AUTOMAKE([-Wno-portability tar-ustar])
 AM_MAINTAINER_MODE([enable])
 
 # Maintainer note - comment this line out if you plan to rerun