This fixes #19814: Lua interface broken after Lua 5.5 upgrade (on Linux).

I used AI. I submitted the problem to OpenCode Zen and this is what it found:

Lua 5.5 renamed the function luaL_openlibs to luaL_openselectedlibs. The vim source code in src/if_lua.c still references the old function name:

• Line 218: # define luaL_openlibs dll_luaL_openlibs
• Line 334: void (*dll_luaL_openlibs) (lua_State *L);
• Line 442: {"luaL_openlibs", (luaV_function) &dll_luaL_openlibs},
• Line 2573: luaL_openlibs(L); // core libs

Verification via nm confirms the symbol doesn't exist in Lua 5.5:

$ nm -D /usr/lib/liblua.so.5.5.0 | grep luaL_openlibs
(nothing returned - symbol renamed)

OpenCode Zen patched if_lua.c and added conditions to test the lua version. I tried the patch on two configurations I have and it was successful:

1. Arch Linux with lua 5.5
2. Ubuntu 24.04 LTS with lua 5.4

I couldn't test on other configurations, in particular MacOS, Windows, BDS, Linux ARM etc.