Suppor for different ABIs and platforms

Hi Moritz,

This is not actually a pull request. I just wanted to check-in if you want that functionality integrated into your repository. Please have a look at the last 2 commits. They add support to choose the ABI (one of armeabi, armeabi-v7a, x86) and the platform (android-8, android-9, android-14). Currently this only works for boost 1.51.0.

If you don't want that extra functionality in your repository, I just maintain it in my own. If you like it and want to merge it, I would clean it up and add the multiple abi/platform support for boost 1.53.0 too.

Let me know what you think.