Update libsvm to 3.19, add --with-tools option

There are two commits. The first just updates the package to the newest version, the other add a `--with-tools`-option. The new options adds the tools that are included in the libsvm package if the flag is set.
`depends_on` goes outside of the install method. So you want something like

```rb
depends_on "gnuplot" if build.with? "tools"
```

near where you added `option "with-tools"`.

`depends_on "python"` should not be necessary, unless this \*requires* the Homebrew python, in which case you want `depends_on :python`.