Lua 5.2.4's `os.execute()` calls `system()`, unavailable on iOS

9. Lua 5.2.4's os.execute() calls system(), unavailable on iOS

What happened:

loslib.c:82:14: error: 'system' is unavailable: not available on iOS

Initial (wrong) assumption: that -DLUA_USE_IOS (which we'd already set) would guard this, the way it seemed to elsewhere in Lua.

What we actually found, by checking Lua's published source across versions directly: the LUA_USE_IOS-aware guard around system() (an l_system macro in loslib.c) was only added in Lua 5.4. This project pins Lua 5.2.4 (see depinfo.sh), and 5.2.4's loslib.c calls system(cmd) unconditionally with no iOS-awareness at all.

Why we can't just upgrade Lua to fix this: mpv's own FAQ states explicitly that mpv does not and will not support Lua 5.3 or newer — only 5.1, 5.2, or LuaJIT. So "upgrade to 5.4" isn't an available option here.

Fix: rather than patching Lua's own source, lua.sh force-includes (-include) a small generated header that #undefs and redefines the system macro to a harmless stub (return cmd ? -1 : 0, matching system(NULL)'s own "no command processor available" convention) before loslib.c's reference to it is ever compiled. os.execute() calls from any Lua script become a no-op reporting failure, rather than the build refusing to compile. No mpv default script actually calls os.execute(), so this has no practical runtime impact for normal playback.

Lesson: a macro that "should" guard something based on its name isn't guaranteed to — verifying against the actual version in use (not the latest version's behavior) mattered here, since the fix upstream added in a later release doesn't retroactively apply to the older, still-in-use version this project depends on.