add android build

添加对android的编译支持 http://wendal.net/2014/05/13.html ,但只针对32位linux下的编译.

```
CC=/opt/ndk-armv7/bin/arm-linux-androideabi-gcc CXX=/opt/ndk-armv7/bin/arm-linux-androideabi-g++ TARGET_OS=OS_ANDROID_CROSSCOMPILE make
```