gcc >= 4.3 is not supported

Building kernel 2.6.16 with gcc 4.3 is completely untested, and
you might run into both kernel and gcc problems (as always with
new gcc versions).

For making this obvious the kernel build now #error's when trying
to build with gcc >= 4.3.

The kernel might work fine when compiled with gcc 4.3 and it's
therefore possible to remove the #error, but if someone really
longs for regressions he can as well try a more recent kernel
instead.

Signed-off-by: Adrian Bunk <bunk@kernel.org>