[RFC] Free config.h from vim.h

config.h was wrapped in vim.h with changes that would not allow the file to be included elsewhere in the project.  This change contributes to the goal of #549.
The purpose of `HAVE_CONFIG_H` is to check `config.h` existence before including it. Moving it to `config.h.in` itself doesn't make sense.

This code should be refactored in another way.