New formula: makeheaders

A tiny but powerful utility from the sqlite and fossil folks (it's used by both projects) for more scalable C/C++ codebases (read: cheaper to maintain), in the flavor of source-also-declares-interface--style development, vaguely similar to Go (Golang).  `makeheaders` is the **source** preprocessor that generates headers that makes the magic happen.  Also, it can generate the externally-exported library header too (`-H` instead of `-h`)! 

The github packaging makes it available to everyone that likes versioning / automation, i.e., a package manager and/or configuration management.