fix bundled libtom configure flag

[Matt Johnston]

On Sat, Oct 12, 2013 at 04:37:15PM -0400, Mike Frysinger wrote:
> the current flag treats --disable-bundled-libtom like enable.  this patch fixes it.

I've committed this modified. If it can't find system libraries
with --disable-bundled-libtom it fails rather than silently
falling back to bundled.

https://secure.ucc.asn.au/hg/dropbear/rev/4cf61252dfc3

Cheers,
Matt