Fix "make dist" of modified git checkouts.

Fix "make dist" of modified git checkouts.

This fixes error messages such as
tar: gettext-0.19.8.1.74-72e49-dirty/gettext-tools/gnulib-tests/test-term-ostream-xterm-linux-mandriva.out: file name is too long (max 99); not dumped
tar: gettext-0.19.8.1.74-72e49-dirty/gettext-runtime/intl-csharp/doc/GNU_Gettext_GettextResourceManager.html: file name is too long (max 99); not dumped

* configure.ac: Pass option 'tar-ustar' to AM_INIT_AUTOMAKE.
(am__tar): Use 'tar' option --format=ustar.