Added extra clapack and cblas header check to extconf.rb, which allows compile on ubuntu

(Fix for #88.)

This commit lets you compile error-free on Ubuntu 13.04 with ATLAS from the package repository using:
```
sudo apt-get install libatlas3-base libatlas-base-dev
bundle install
rake compile
```
without having to set any environment variables or anything else special.  (Though if lapack is installed separately from ATLAS, it will still cause problems when trying to load nmatrix in a program.)

I only have access to Ubuntu 13.04 right now, so I haven't tested other versions.  To those out there with other Ubuntu versions: does this fix the issue?  To those out there with other linux distros or OS X: does this break anything?