Description of problem:
Add '-fwrapv' to gcc CFLAGS. This makes sure that gcc doesn't try to optimize
away wrapping arithmetic, which the kernel occasionally uses for overflow
testing, ie things like

if (ptr + offset < ptr)

which technically is undefined for non-unsigned types. See

http://bugzilla.kernel.org/show_bug.cgi?id=12597

for details.

Not all versions of gcc support it, so we need to make it conditional (it looks
like it was introduced in gcc-3.4).

This bug is filed to address the mrg-1.2 kernel. This is not a dup of bug 491264.