Add -Werror=declaration-after-statement to CFLAGS for gcc/clang

From #340:
>Something to consider in the future might be a move over to the 'waf' build system, primarily because it greatly simplifies the ability to use MSVC (as well as other compilers) in a CI environment. Not really a priority right now, but I think down the road that would be really useful. Unlike the mozilla build system, it's clever enough to get a workable MSVC build-system by itself, without needing to open a child shell with the temporary environment variables set up. It is also able to generate MSVC, Xcode and Eclipse projects, so it's great for debugging. I have a sample waf buildsystem project I've used for testing it, at http://github.com/caitp/waftest. It's still a bit clumbsy, but it's pretty simple to migrate over to