bugfix: LuaJIT library auto-discovery regression

[dylanplecki]

This change fixes the LuaJIT library auto-discovery build regression from a96e99a and adds additional LuaJIT auto-discovery for LuaJIT-2.1.

Fixes #1661.

I hereby granted the copyright of the changes in this pull request to the authors of this lua-nginx-module project.

[thibaultcha]

This PR reminds me a lot of #1577 which wasn't accepted, alas.

[dylanplecki]

Thanks for the insight @thibaultcha.

Hopefully, we get this regression fix through (or some version of it, #1577 looks interesting). It will impact LuaJIT auto-discovery in the main Nginx Debian repo in the future, though they're currently pinned to an older version of http-lua for now to support non-LuaJIT builds on odd arches.

Anecdotally, I ran into this bug while attempting to update an internal Nginx Debian repo to Nginx v1.19.2 and http-lua v0.10.17, with the regression taking place after v0.10.16. The package maintainers have a workaround patch for v0.10.13 but will break again after http-lua v0.10.16.

[thibaultcha]

@dylanplecki Since so many efforts have been invested into optimizing OpenResty's LuaJIT fork for OpenResty's use-cases, it is highly advised to run one of the formal OpenResty releases. It is especially more so the case when considering the Nginx and OpenSSL patches needed by OpenResty, as well as the lack of testing for ngx_http_lua on some of these Nginx versions. Can't you use a formal OpenResty release instead?

[thibaultcha]

It seems I should have asked this question in #1661 instead.