Add script for determining the set of symbols to export from a library. * build-aux/declared.sh: New file, from GNU libunistring. * doc/lib-symbol-visibility.texi (Exported Symbols of Shared Libraries): Mention it.

#!/bin/sh

./test-ftell${EXEEXT} 1 < "$srcdir/test-ftell.sh" || exit 1
echo hi | ./test-ftell${EXEEXT} || exit 1
exit 0