Use directory-exists check rather than file-exists check

Hi John,

The `aclocal --print-ac-dir` command in `bootstrap.sh` returns a directory so I believe the existence check a couple of lines later needs to be against a directory rather than a file.

This was preventing compilation from source on an Ubuntu 13.04 box. I was seeing errors like:

cp: cannot stat /usr/local/share/aclocal/codeset.m4: No such file or directory
cp: cannot stat /usr/local/share/aclocal/gettext.m4: No such file or directory
cp: cannot stat /usr/local/share/aclocal/glibc21.m4: No such file or directory
cp: cannot stat /usr/local/share/aclocal/iconv.m4: No such file or directory

This change fixed it.