memset suggestion.

[Ben Lindstrom]

Since 3.x is rolling out with -fno-builtin-<comand> support maybe we
should test for 3.x and set -fno-builtin-memset which should stop gcc from
optimizing it away.  And maybe everyone should look for the same feature
in their favorite compiler.

There is not much OpenSSH can do about bad compilers optimization, but
that is the only reasonable way to stop it starting in the 3.x series of
gcc.

- Ben