extensions: install the lua interpreter for all non-relocatable builds.

As discussed in #11876, the interp files work fine on Linux (and other
platforms) too, and this allows to properly redirect to the working
interpreter found at configure time, without touching the scripts'
shebang.

extensions/lua.interp.in

@@ -1,5 +1,5 @@
-lua=@LUA_PATH@luajit.exe
-luajit=@LUA_PATH@luajit.exe
-/usr/bin/luajit=@LUA_PATH@luajit.exe
-/usr/bin/lua=@LUA_PATH@luajit.exe
-:Lua:E::lua::luajit:
+lua=@LUA_PATH@
+luajit=@LUA_PATH@
+/usr/bin/luajit=@LUA_PATH@
+/usr/bin/lua=@LUA_PATH@
+:Lua:E::lua::@LUA_PATH@:

extensions/meson.build

@@ -1,15 +1,19 @@
 subdir('goat-exercises')
 
-# Fallback fix to the problem of non-configured interpreters
-if platform_windows and not meson.is_cross_build() and lua.found()
+if not meson.is_cross_build() and lua.found() and (platform_windows or not relocatable_bundle)
   lua_config = configuration_data()
-  lua_config.set('LUA_PATH', '')
+  # For Windows, we set the binary name only.
+  # For other OSes, we use the full path (when not in a relocatable
+  # bundle).
+  if platform_windows
+    lua_config.set('LUA_PATH', lua_bin + '.exe')
+  else
+    lua_config.set('LUA_PATH', lua.full_path())
+  endif
 
-  configure_file(
-    input : 'lua.interp.in',
+  configure_file(input : 'lua.interp.in',
                  output: 'lua.interp',
                  configuration: lua_config,
                  install: true,
-    install_dir: gimpplugindir / 'interpreters',
-  )
+                 install_dir: gimpplugindir / 'interpreters')
 endif