use latest lwip sources, fix gcc warnings

[rapgro]

see https://savannah.nongnu.org/git/?group=lwip

[michaelblyons]

I am not the repo owner (or even a contributor) so take this with a shaker of salt:

If syncing to the latest lwip is important, would you consider converting it to a submodule? Then, reviewing this change doesn't drag one through 600 changed files. The lwip section of this project pre-dates Git submodules, and I think it's a prime example of their foremost purpose.

[rapgro]

Agreed, with a submodule it seems the proper way to go. But when you read the documentation provided from the lwip project, they suggest to bundle the sources in the client application(s). The license is BSD, so forks shouldn't be of any issue at all. No idea what internals like API changed with the new sources.
All in all, if someone can refactor the sources to have a proper API with a submodule, I'd be the last guy against such a progress.

[rapgro]

The main reason why I decided to build with newer lwip sources was taken because of several gcc warnings I wanted to fix to be able to provide good packaging (Fedora).