meson: Prevent broken lua plug-ins

See: https://gitlab.gnome.org/GNOME/gimp/-/issues/11876

For 3.0 we will support only Lua 5.1, since we need to figure out how
much of the Lua 5.1 API is compatible with newer versions (Lua minor
versions aren't like Python minor versions, there is API breakage so
plug-ins would be completely unreliable, this commit prevent this).

Also, change some files to not force luajit since it isn't mandatory.

INSTALL.in

@@ -291,7 +291,7 @@ help in that regard:
      "gegl:matting-levin" GEGL operation for alternative matting engine
      Python @PYTHON3_REQUIRED_VERSION@ and PyGObject for Python 3 plug-ins
      GJS for Javascript plug-ins
-     LuaJIT and LGI for Lua plug-ins
+     Lua 5.1 (or LuaJIT) and LGI for Lua plug-ins
      dot for "Show Image Graph" (unstable branches)
      xdg-desktop-portal implemented for your desktop for various D-Bus API (screenshot, color-picking…)
 

extensions/goat-exercises/goat-exercise-lua.lua

@@ -1,4 +1,4 @@
-#!/usr/bin/env luajit
+#!/usr/bin/env lua
 
 -- GIMP - The GNU Image Manipulation Program
 -- Copyright (C) 1995 Spencer Kimball and Peter Mattis

meson.build

@@ -1117,12 +1117,12 @@ endif
 ## Lua
 
 have_lua = false
-# At time of writing, lua-lgi works with Lua 5.1, 5.2, 5.3, 5.4 and LuaJIT2. On
-# most platforms, we use luajit.
+# At time of writing, lua-lgi works with Lua 5.1, 5.2, 5.3 and LuaJIT2, but we
+# support only Lua 5.1 API. See: https://gitlab.gnome.org/GNOME/gimp/-/issues/11876
 if get_option('lua').allowed()
   have_lua_lgi = false
 
-  foreach lua_bin : [ 'luajit', 'lua5.1', 'lua5.2', 'lua5.3', 'lua5.4', 'lua' ]
+  foreach lua_bin : [ 'luajit', 'lua5.1', 'lua' ]
     lua = find_program(lua_bin, required: false)
 
     if lua.found() and meson.can_run_host_binaries()