Specify CMAKE release type in proj-sys?

Cmake has four build types:

> Choose the type of build, options are: None (default), Debug, Release,
RelWithDebInfo, or MinSizeRel

We should be producing a release build of libproj when building from source, and it's not clear to me whether we currently are. It is able to be set by passing `config.define("CMAKE_BUILD_TYPE", "Release");` but I need to figure out whether this actually produces a release artifact.

Start in proj-sys and inspect how the source build config is assembled, especially the config.define("CMAKE_BUILD_TYPE", "Release") setting. Build from source and verify whether the resulting libproj artifact uses the Release configuration; done means the release type is explicitly specified and its effect is confirmed.

Written by the indexing model from the issue text.