bugfix: config: ensured LuaJIT FFI check and static PCRE builds succeed on Darwin.

[thibaultcha]

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

This ports a couple of bugfixes from #1577 into the current version of the config script.

Fixes are:

• fix the LuaJIT FFI check (tested on Linux and Darwin, ideally should be tested on Windows too)
• fix static libpcre builds on Darwin