Added dir_configs for all libs

Like I said in [issue #60](https://github.com/SciRuby/nmatrix/issues/60), `dir_config` would allow people to use --with-lib-dir, --with-lib-include and --with-lib-lib with atlas, lapack, clapack and cblas.

We could also try to improve the lists of "reasonable defaults" for the location of each one, too.

This way, it should be dead simple to install NMatrix. With everything in your system, it's only a matter of `gem install -- --with-lib-etc=...`, giving the correct pathnames.