* [jvm-packages] Fixed JNI_OnLoad overload It does not compile on Windows without proper export flags. * [jvm-packages] Use JNI types directly where appropriate * Removed lib hack from CMake build Prior to this commit the CMake build use hardcoded lib prefix for libxgboost and libxgboost4j. Unfortunatelly this did not play well with Windows, which does not use the lib- prefix.