run bootstrap if .gnulib is not present

run bootstrap if .gnulib is not present

If .gnulib is deleted unexpectedly, autogen.sh will fail with message:

fatal: ambiguous argument '.gnulib': unknown revision or path not in the working tree.
Use '--' to separate paths from revisions, like this:
'git <command> [<revision>...] -- [<file>...]'

which is actually given by git diff .gnulib, which doesn't exist.

In the case to run bootstrap to create .gnulib.