Use CFLAGS needed to get a working executable for neovim

Neovim is now built using the CFLAGS needed for an actual working release version. The flags used come from this comment https://github.com/neovim/neovim/pull/715#issuecomment-44793621
It is, the problem is it uses D_FORTIFY_SOURCE=2, and that gives an error when you try to run it. There's an issue open for that problem, but it should be solved automatically ofter they finish the refactoring.