FindLua: Set CMP0012 to NEW for the revised search code

Code added by commit v3.12.0-rc1~53^2 (FindLua: Search for lua.h using
more conventional paths, 2018-05-20) depends on `CMP0012` NEW behavior.
Set the policy explicitly for the scope of the FindLua module.

Fixes: #18142

Modules/FindLua.cmake

@@ -36,6 +36,9 @@
 # This is because, the lua location is not standardized and may exist in
 # locations other than lua/
 
+cmake_policy(PUSH)  # Policies apply to functions at definition-time
+cmake_policy(SET CMP0012 NEW)  # For while(TRUE)
+
 unset(_lua_include_subdirs)
 unset(_lua_library_names)
 unset(_lua_append_versions)
@@ -236,3 +239,5 @@ FIND_PACKAGE_HANDLE_STANDARD_ARGS(Lua
                                   VERSION_VAR LUA_VERSION_STRING)
 
 mark_as_advanced(LUA_INCLUDE_DIR LUA_LIBRARY LUA_MATH_LIBRARY)
+
+cmake_policy(POP)