docs: mention that not all values can be exported

There has been a LOT of news about bash's Shell Shock bug lately.
Document some of the ramifications it has on portable scripting.

* doc/autoconf.texi (Limitations of Builtins) <export>: Add some
details about Shell Shock CVE-2014-6271.

Signed-off-by: Eric Blake <eblake@redhat.com>
tests: avoid spurious test failure with libtool 2.4.3

Based on a report by Bruce Dubbs.

* tests/foreign.at (Libtool): Be tolerant of 'quote' replacing the
older `quote'.

Signed-off-by: Eric Blake <eblake@redhat.com>