Ship GEOS with the project for MRI users

[BuonOmo]

The V3 roadmap includes our goal of having an easier installation process.

Since our pure ruby implementation is incomplete, and our the FFI version is way slower than the Geos CAPI version, it is clear that we want to keep the CAPI, and make it easier to install if we can.

Some idiomatic projects (such as https://github.com/sass/sassc-ruby) are showing us a good way to operate: intregrate GEOS directly in the CAPI build. There are tons of other projects that are doing so, you can just search github for those.

Once we have a fixed version of GEOS shipped with RGeo, we also want one to be able to still be able to install its custom GEOS version.

One last thing, including GEOS version in our version, or a helper would be nice... Think SELECT postgis_full_version() for RGeo.

I am aware that this task may be a challenge for a first contributor. However, if you want to contribute, we'll be there to work together 💪

[BuonOmo]

Note on advancement, there is #291 for binding another library than geos-config, and there is https://github.com/buonomo/test-geos for downloading and installing geos in extconf.rb

[BuonOmo]

Current state:

1. we are still wondering how this would end up working
2. the dreamed state would be to ditch the pure ruby implementation in favor of the geos one, since it can come along
3. we like pg's extconf.rb and way to handle that
4. we may want to cache some pre-compiled versions at the rubygems level to have a quicker installation for users

[eis-ioki]

I would love to see rgeo brings its own geos automatically, just like nokogiri brings its own libxml2: https://nokogiri.org/#cruby

[BuonOmo]

@eis-ioki thanks for the feedback! Do you want to contribute on that topic?