Shims required to support GEOS version-specific functions in Cython

[brendan-ward]

The standard approach of declaring an entry for a GEOS function in _geos.pxd does not work when the function is not available in a given older version of GEOS.

In the C extension, we are able to use macros like GEOS_SINCE_3_7_0 to conditionally include these for the compiler.

I don't see a similar mechanism for Cython.

It looks like other projects (e.g,. Fiona) deal with this by defining shim *.pxd files that are dynamically selected and used for the build based on the targeted version of the C library. I'm not yet sure how those handle functions that were introduced into later versions of the C library.

Is there a better alternative approach?

[snowman2]

rasterio/rasterio#2016

[snowman2]

Another example: Toblerity/Fiona#979

[brendan-ward]

Thanks @snowman2 !

One challenge here compared to rasterio / Fiona is that we don't require an environment variable indicating GEOS version when geos-config is not available (e.g., on Windows?). However, it might be a good idea for us to do so in order to check and enforce minimum GEOS version, and then using compile_time_env in setup.py becomes straightforward.