Moved LINK_FLAGS for MinGW into macro definition

  * Moved
    if(CMAKE_COMPILER_IS_GNUCXX AND MINGW)
      set_target_properties(${_exename} PROPERTIES LINK_FLAGS "-Wl,--allow-multiple-definition")
    endif()
  logic to PCL_ADD_EXECUTABLE, PCL_ADD_EXECUTABLE_OPT_BUNDEL and PCL_ADD_TEST.