Fixes: LoadError: /unicorn_http.so: undefined symbol: Init_unicorn_http

[NielsKSchjoedt]

Summary

• teach ext/unicorn_http/extconf.rb to regenerate unicorn_http.c via Ragel when the generated parser is missing, accepting absolute --with-ragel values and otherwise searching PATH
• fail fast with an actionable error message when Ragel cannot be located so git installs don’t ship an extension without Init_unicorn_http
• document the Ragel requirement and extconf check for anyone installing unicorn straight from git

Testing

• ⚠️ ruby ext/unicorn_http/extconf.rb