refactor: many improvements to the NGINX module config script.

This is a major refactor of the `config` script of this module.

* The script is now compatible for both ngx_http_lua and ngx_stream_lua
* ngx_sream_lua can now be compiled as a dynamic module
* The script is able to skip many steps if another Lua module has
  previously been configured, which avoids many redundant operations and
  duplicated compilation flags
* The script now automatically detects source files in the `src/`
  directory
* Large refactor for improved maintainability
* Force `pcre_version` symbol
    * Fixed an issue on Darwin for static PCRE builds
    * Require the symbol to be defined for linking
    * Smarter linker flags detection depending on platform
* Require LuaJIT with FFI
    * Fixed an issue on Darwin which prevented the test form ever
      succeeding; it now statically links before running to avoid an
      issue with the .dylib location
* Align error messages with NGINX core's style (no indentation)
* Added a vi modeline

Author: thibaultcha