message: cmake: don't try to build for iOS on Linux
diff: # To cross compile for iPhone:
  # build$ cmake .. -DCMAKE_TOOLCHAIN_FILE=../contrib/cmake/CrossCompile-iOS.cmake
  +if (NOT APPLE)
  + message(FATAL_ERROR "\nNeed to build on a Mac for iOS\n")
  +endif()
  +
  SET(CMAKE_SYSTEM_NAME Generic)
  if (NOT "$ENV{IOS_SDK_VERSION}" STREQUAL "")
  SET(CMAKE_SYSTEM_VERSION $ENV{IOS_SDK_VERSION})
  