Specify CMAKE release type in proj-sys?

[urschrei]

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.