Windows support using CMake

[zachasme]

This PR builds the entire project using CMake instead of PGXS.

I think this is a good idea since CMake is already a hard dependency from H3, and it makes it easier to build and install h3-pg on both Windows and MacOS.

There are still a few things I'd like to figure out:

• whether we can build against debug variants on some programs (like it appears upstream is doing https://github.com/Kitware/CMake/blob/master/Modules/FindPostgreSQL.cmake#L200)
• which _INCLUDE_DIR from pg_config to use
• how to make sure inlining works on mac

[zachasme]

I've been working on this for a while, as part of providing h3-pg for a Windows user, and I think it is ready for testing and/or input if anyone has views on this rather large change.

@Komzpa, @mngr777 do any of you guys have any thoughts on this? I'm hoping to wrap it in the Makefile so it should keep working with pgxn. How are you currently installing h3-pg?

[Komzpa]

@zachasme we use pgxn install h3 on linux boxes.