参考：
http://stackoverflow.com/questions/6849981/where-do-i-find-old-versions-of-android-ndk
地址：
1.windows
https://dl.google.com/android/ndk/android-ndk-r6-windows.zip
https://dl.google.com/android/ndk/android-ndk-r6b-windows.zip
https://dl.google.com/android/ndk/android-ndk-r7c-windows.zip
https://dl.google.com/android/ndk/android-ndk-r8e-windows-x86.zip
https://dl.google.com/android/ndk/android-ndk-r8e-windows-x86_64.zip
https://dl.google.com/android/ndk/android-ndk-r9d-windows-x86.zip
https://dl.google.com/android/ndk/android-ndk-r9d-windows-x86_64.zip
2.mac
https://dl.google.com/android/ndk/android-ndk-r7c-darwin-x86.tar.bz2
https://dl.google.com/android/ndk/android-ndk-r8e-darwin-x86_64.tar.bz2
https://dl.google.com/android/ndk/android-ndk-r9d-darwin-x86_64.tar.bz2
3. linux
https://dl.google.com/android/ndk/android-ndk-r8e-linux-x86_64.tar.bz2
https://dl.google.com/android/ndk/android-ndk-r9d-linux-x86_64.tar.bz2
