Howto default to release build with a cpp-application

I’ve a cpp-application build.gradle. When I call ./gradlew assemble, it builds this applcation in debug mode.
But I want it to be build in release mode and build the application when needed via assembleDebug in debug mode.

How is this possible to configure?
I found some discussions about ‘defaultPublishConfig “release”’ but can’t find the right place in my build.gradle to set this property.

Would this be the right way to do what I want to do? Where does this need to be set? I don’t build for android, so I’ve no android section!