public void initializeGlobalExtensions() {
    for (Class<? extends IGlobalExtension> clazz : globalExtensionClasses) {
      verifyGlobalExtension(clazz);
      IGlobalExtension extension = instantiateAndConfigureExtension(clazz);
      globalExtensions.add(extension);
    }
  }
Maintains a registry of global Spock extensions and their configuration objects,
which can be used to configure other extensions.

@author Peter Niederwieser
spock-core/src/main/java/org/spockframework/runtime/GlobalExtensionRegistry.java