Modified extconf to use shared snappy library if available.

I ran into an issue installing the snappy gem in our staging/production environments.  Due to firewall rules, the extconf script could not download the snappy-1.1.1 source from google code, which caused the native extensions to not compile.

The native snappy libraries are already installed on our servers and I modified extconf to pick those up if they're available.  I made sure the tests still pass on systems that do not have the native lib installed.  I haven't worked with native extensions much, so please let me know if you have any concerns.