Add another search path for system dependencies [Ubuntu 12.04]

This was my solution for the problem in #244. I'd installed dependencies using apt-get according to the installation instructions but I wasn't getting jpeg/gif support. has_lib.sh wasn't finding anything.

This may or may not be a proper solution for this. Perhaps the test should recurse rather than add a specific sub directory? I feel like I'm out of my league here but I'd love to save someone else this hassle.