fix bugs in building Java Wrapper in Windows env

'CP' is not a valid command in native Windows, which will cause failure in building Java Wrapper. Use 'COPY' instead and correct the format of file path correspondingly

CMakeLists.txt

@@ -494,7 +494,7 @@ if(USE_SWIG)
  TARGET _lightgbm_swig
  POST_BUILD
  COMMAND "${Java_JAVAC_EXECUTABLE}" -d . java/*.java
- COMMAND cp "${PROJECT_SOURCE_DIR}/Release/*.dll" com/microsoft/ml/lightgbm/windows/x86_64
+ COMMAND copy "..\\Release\\*.dll" "com\\microsoft\\ml\\lightgbm\\windows\\x86_64"
  COMMAND "${Java_JAR_EXECUTABLE}" -cf lightgbmlib.jar com
  )
  endif()