The libaes_siv dependency

[Eric S. Raymond]

I've added a mandatory waf check for the libaes_siv library.
The requirement for it has been document it in INSTALL for some time;
it's required for the NTS Support we're developing.

We didn't source-include it because Daniel wants his cmake build to
keep control of the compilation environment for the library.

At some point it will probably be taken into OpenSSL and this separate
dependency will go away.
--
						>>esr>>