build: Fallback $docdir to $datarootdir/doc/gettext
Problem reported by Santiago Vila in
<http://lists.gnu.org/archive/html/bug-gettext/2014-05/msg00014.html>.
* gettext-runtime/configure.ac: Adjust $docdir assuming that
AC_PACKAGE_TARNAME is set.
* gettext-tools/configure.ac: Likewise.
* gettext-tools/examples/configure.ac: Likewise.
