Botan 1.11 / C++11 support

Botan 1.1/C++11 patch. Note the Botan 1.11 on some systems (my Linux Fedora 19 64 bit VM for instance) crashes on a destruction order (locking allocator destroyed too soon) which is not yet fixed.
A simple solution is to disable it as it is for OS X (aka darwin).
Anyway the C++11 part is interesting by itself.