Use configure test for aligned attribute.

* configure.ac (HAVE_GCC_ATTRIBUTE_ALIGNED): New test and ac_define.
* cipher/cipher-internal.h, cipher/rijndael.c, random/rndhw.c: Use new
macro instead of a fixed test for __GNUC__.

We assume that compilers that grok "attribute ((aligned (16)))"
implement that in the same way as gcc does. In case it turns out
that this is not the case we will need to do two more things: Detect
such different behaviour and come up with a construct to allows the
use of that other style of alignment forcing.