scons/lua.py makes use of the vestigial luadir option from commit
e94dcecf17.
Like FindLua.cmake, scons/lua.py searches for the Lua headers and
library, instead of using pkg-config like the old scons/lua.py (removed
in commit 9929d3ca1c) did, because even though distributions typically
provide .pc files for Lua, upstream Lua doesn't. It's likely that all
distributions that compile Lua as a C++ library will also provide .pc
files, but this check doesn't rely on that (just as the CMake module
doesn't).
Unfortunately, SCons.Conftest.CheckLib() prints up to eight messages
like "Checking for C++ library lua54-c++... no" until a working library
name is found.
Also conditionally include system Lua headers in src/lua/*.h and update
documentation in src/modules/lua_README.md, src/wesnoth_lua_config.h,
and src/wesnoth_lua_config.md. The two lines about "The primary commit,
after replacing the sources," in src/wesnoth_lua_config.md don't make
sense since the instructions were updated for submodule Lua in commit
d32cfb88c4 and make even less sense now with preceding commits for
updating CMake modules.
