using \undef instead of libc

With this:

https://github.com/ghedo/p5-FFI-Raw/pull/33

we can now pass in undef as the first argument to `FFI::Raw->new` to search for symbols in the current process (even on windows), and thus you can search for libc symbols without having to know the name of libc.

I am making this a pull request, but I think that actually what would be better would be to provide some more friendly interface for this capability, maybe a variant of `lib_ffi`, that takes no arguments and inserts `undef` into the search list.