add static feature for building geos-sys with statically linked libgeos

[gauteh]

Depends on georust/geos-sys#21

[GuillaumeGomez]

I cherry-picked your commit into #100. I'll make releases very soon.

[GuillaumeGomez]

New versions have been published.

[gauteh]

Thanks. I think that the exclude here: https://github.com/georust/geos/blob/master/sys/geos-src/Cargo.toml#L11 prevents the source from being checked out. You could potentially skip other large parts of the source tree if it is a problem: https://github.com/alexcrichton/openssl-src-rs/blob/main/Cargo.toml#L11