Using wuffs-c (not wuffs) is awkward

The `wuffs` command line tool is used when working on Wuffs-the-Library. The `wuffs-c` command line tool is (implicitly) part of that, but it can also be used on its own for other code written in Wuffs-the-Language, including code that doesn't live in this repository.

Doing that works... sort of. But it's not a great experience. The `hello-wuffs-c` example has to declare a placeholder `wuffs-base.c` file (as it can't rely on the generated `gen/c/wuffs-base.c` file), and the C driver program has to `#define WUFFS_CONFIG__MODULE__BASE`.

Both of these workarounds shouldn't be necessary.