Fix build errors with multiple Lua libraries

[stanhu]

This commit fixes two items:

1. Previously configure would search for Lua in known paths, but
   this could cause the Makefile to use inconsistent header and library
   paths. For example, if luajit were installed in /usr/local/include
   but lua 5.1 were installed in /usr/lib/liblua-5.1.so, the build
   would attempt to use the luajit headers but link against the lua 5.1
   library.

To fix this, we switch the order of the search:

• First attempt to find an installed LUA library with pkg-config.
• If we cannot find a library that way, fall back to the known-path
  scan.

This is actually what is already documented in LUA_POSSIBLE_PATHS.

Note that PKG_CONFIG_PATH can be specified in configure to look in the right path:

PKG_CONFIG_PATH=/usr/local/pkgconfig ./configure

2. Add luajit back into LUA_POSSIBLE_LIB_NAMES. This was added in
   0ac23a4 but quietly reverted in fe98ce4. The changes in the first
   item also ensure the CFLAGS are set properly for luajit.

This should fix the issues raised in #1909.

[stanhu]

@victorhora Could you take a look at this?

[stanhu]

@martinhsv Would you be able to review this?

[martinhsv]

Hello @stanhu ,

The only question that comes to mind initially is about where you have placed the two new items in LUA_POSSIBLE_LIB_NAMES.

I guess I'm wondering aloud if it might be better for the two luajit names to be at the end of the list instead. That might make the order in which things are looked for more straightforward (I.e. all regular lua options first, then luajit)

[stanhu]

@martinhsv I moved them to the end. It's been a while since I looked at this, so I don't remember if there was a reason why I put them before the standard lua options. But the main issue as I recall was that we weren't taking advantage of pkg-config to find the right libraries and headers when there was ambiguity.

[stanhu]

From #1909 (comment), I think I intended to the luajit modules to precede lua since luajit could install itself in /usr/local/lib/lua.