#13 pentium2 build can use optimised lattice siever code

[Geoffrey Reynolds]

The pentium2 build currently uses the generic code
for the lattice siever. This patch allows it to make
use of the lasieve4/piii siever configuration. It
depends on the compiler defining __SSE__
appropriately, GCC does, but I don't know if this is
a standard symbol for other compilers.

I regularly use P2 machines with the lattice siever
and pol51 tools built like this. Although I wouldn't
normally do linear algebra on a P2, I have tested it
with tests/snfs_small and it worked fine.

[Geoffrey Reynolds]

Patch to make pentium2 build use optimised lattice siever code