Support for boost-1.51.0

I based the patch on the files from 1.53.

Please note that I removed some defines from user.hpp which were already defined via the user-config.jam file.

Furthermore, I removed the android.py and android.jam files from the patch. gcc.jam is used for compilation and these files are (now) obsolete. At least it looks that way to me. Let me know if that's correct.