[build] Use tar-ustar instead of tar-pax

The default tar-v7 is not good enough for us (99 char filename
limit), so I have had bumped to tar-pax.  We got one complaint
that someone's tar couldn't handle tar-pax.  Set to tar-ustar
which is ~13 years earlier than tar-pax and is good enough for us.
diff --git a/configure.ac b/configure.ac
index 91ccb70..7a7f8ac 100644
--- a/configure.ac
+++ b/configure.ac
@@ -9,7 +9,7 @@
 AC_CONFIG_SRCDIR([src/harfbuzz.pc.in])
 AC_CONFIG_HEADERS([config.h])
 
-AM_INIT_AUTOMAKE([1.11.1 gnits tar-pax dist-bzip2 no-dist-gzip -Wall no-define color-tests -Wno-portability])
+AM_INIT_AUTOMAKE([1.11.1 gnits tar-ustar dist-bzip2 no-dist-gzip -Wall no-define color-tests -Wno-portability])
 AM_CONDITIONAL(AUTOMAKE_OLDER_THAN_1_13, test $am__api_version = 1.11 -o $am__api_version = 1.12)
 AM_SILENT_RULES([yes])