If DOWNLOAD_RUBY is set to OFF, the build will not attempt to download
a prebuilt libruby.so and not fire a warning that one is not present.
This may be desired if one wants to link dfhack against a newer system
libruby.so, now that dfhack supports ruby 2.x and also supports linking
against system libruby (at least, on Linux), as per the following commit:
df9b5bca73
This also allows the dfhack build to proceed without network access
(once all submodules have been fetched), which at least some Linux
distributions, like Fedora, require.
By default DOWNLOAD_RUBY is set to ON, so this should not change
the behavior of default builds.
I tested that when DOWNLOAD_RUBY is set to OFF, dfhack still launches
and that the Ruby plugin still functions, e.g. by running
```rb_eval puts('Hello world.')``` in the dfhack console.