WIP: Automatically create per-source header files

Currently, we output symbol definitions into every generated .c file, unless the user specifies a header file. Unfortunately, this leads to ugly C code, and fixing that was a major goal of #713 (closed). In working on that issue, I misinterpreted and wrote a patch that will always generate header files regardless of the compiler options.

With this patch we would generate one "true" header file and symlink the others together. This is a little crude, but it works well, since we don't want to mess with filenames on other people's systems. This cleans up the .c sources regardless of whether or not a header file is specified.

Alternative approaches may also be viable, such as generating different header files for every source file. So this is a speculative merge request.

FIXME: currently the commits are all out of order and I need to rebase. I need to review the git manual to do it correctly.