[Rivet] PATCH: Don't re-download boost tarball if re-running the bootstrap script

[David Bjergaard]

Hi,

Here's a patch to avoid unnecessarily downloading multiple copies of the
boost tarball if the rivet-bootstrap script fails for some other reason:

75c75
<     wget http://downloads.sourceforge.net/boost/boost_$BOOST_VERSION.tar.gz
---
>     [ ! -f boost_$BOOST_VERSION.tar.gz ] && wget http://downloads.sourceforge.net/boost/boost_$BOOST_VERSION.tar.gz

The bootstrap script has much improved since 1.8! Thanks for all the
hard work.

Cheers,

    Dave