Add LuaJIT support to AX_PROG_LUA macro

[alerque]

This is a follow up to #287.

This is a much bigger change adding support for LuaJIT as a Lua interpreter. This is a tricky addition because two sets of version numbers are involved: the Lua VM version (which for LuaJIT is lower that current PUC Lua releases, 5.1 vs 5.4) and also a JIT version. It is also quite common to have both PUC Lua and LuaJIT variants installed on a system, but the lower numbering scheme does not mean that PUC should be preferred.

This implementation leaves it up to the user to specifically request LuaJIT be considered by adding a --with-luajit configure option. If they do that and it is set to true, then the regular Lua macros will function in the usual way but with the binary names, include paths, and other things tweaked for use with LuaJIT.

I've been using earlier iterations of this (based on a much earlier serial number) for several years with SILE on a number of platforms including Arch Linux, Ubuntu, Nix, Homebrew, OpenBSD, and many others. It has gone pretty well so far. This version is a bit cleaned up and rebased on serial 44.