Use tar-ustar to avoid problems with a long path in the gtest directory that…

Use tar-ustar to avoid problems with a long path in the gtest directory that crosses the 99-char tar-v7 limit if the package version number happens to be long enough.  This was causing make distcheck to fail on release candidates, although it worked fine for final releases.

c++/configure.ac

@@ -14,7 +14,7 @@ AS_IF([test "x${ac_cv_env_CFLAGS_set}" = "x"],
 AS_IF([test "x${ac_cv_env_CXXFLAGS_set}" = "x"],
       [CXXFLAGS="-O2 -DNDEBUG"])
 
-AM_INIT_AUTOMAKE
+AM_INIT_AUTOMAKE([tar-ustar])
 
 AC_ARG_WITH([external-capnp],
   [AS_HELP_STRING([--with-external-capnp],