build Lua from vendored source instead of the prebuilt lua51.lib

The tree carried two prebuilt Lua static libraries and a copy of the Lua
public headers. lua51.lib is 5.1.2 and matched the headers; lua51.vc9.lib
is 5.1.4 and was dead weight — it came second in Ja2_Libraries, so the
linker resolved every Lua symbol out of lua51.lib and never pulled an
object from it. Just as well, since it asks for /DEFAULTLIB:MSVCRT while
we build /MT.

ext/lua-5.1.5 is the upstream tarball unmodified, built as lua51 the way
the other vendored libraries are, and its src directory replaces lua/ as
the home of lua.h, luaconf.h, lauxlib.h and lualib.h. Those four headers
were stock 5.1.2 retabbed, so 5.1.5 is bugfix-only against what the game
compiled against; the bytecode format is unchanged across 5.1.x and every
script under gamedir is plain source anyway. lua/lua.hpp had no includers
and returns to etc/ where upstream keeps it.

/SAFESEH:NO goes with it. Its two stated reasons were lua51.lib and the
smackw32 import library, and both are now gone: every remaining prebuilt
static library is SAFESEH-clean (libexpatMT.lib 5 of 5 members with
@feat.00 = 0x1, RakNetLibStatic.lib 79 of 79), and lld-link emits a
3155-entry SEHandlerTable without it.

The /MT comment blamed the wrong library. lua51.lib carried no linker
directives at all; RakNetLibStatic.lib is what pins us to the static
runtime, with /DEFAULTLIB:LIBCMT and /DEFAULTLIB:libcpmt.

Verified by building all four applications in Debug and Release, and by
linking the tarball's own lua.c against our lua51.lib with the build's
clang-cl flags and running it under Wine: 5.1 stdlib, GC, coroutines and
the x86 __asm fld/fistp lua_number2int fast path all behave. The game
itself could not be launched here — this checkout's gamedir has loose
Data directories but no SLF archives, so VFS aborts on Data\Ambient.slf
long before any script runs.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

ext/lua-5.1.5/src/lfunc.h

@@ -0,0 +1,34 @@
+/*
+** $Id: lfunc.h,v 2.4.1.1 2007/12/27 13:02:25 roberto Exp $
+** Auxiliary functions to manipulate prototypes and closures
+** See Copyright Notice in lua.h
+*/
+
+#ifndef lfunc_h
+#define lfunc_h
+
+
+#include "lobject.h"
+
+
+#define sizeCclosure(n)	(cast(int, sizeof(CClosure)) + \
+                         cast(int, sizeof(TValue)*((n)-1)))
+
+#define sizeLclosure(n)	(cast(int, sizeof(LClosure)) + \
+                         cast(int, sizeof(TValue *)*((n)-1)))
+
+
+LUAI_FUNC Proto *luaF_newproto (lua_State *L);
+LUAI_FUNC Closure *luaF_newCclosure (lua_State *L, int nelems, Table *e);
+LUAI_FUNC Closure *luaF_newLclosure (lua_State *L, int nelems, Table *e);
+LUAI_FUNC UpVal *luaF_newupval (lua_State *L);
+LUAI_FUNC UpVal *luaF_findupval (lua_State *L, StkId level);
+LUAI_FUNC void luaF_close (lua_State *L, StkId level);
+LUAI_FUNC void luaF_freeproto (lua_State *L, Proto *f);
+LUAI_FUNC void luaF_freeclosure (lua_State *L, Closure *c);
+LUAI_FUNC void luaF_freeupval (lua_State *L, UpVal *uv);
+LUAI_FUNC const char *luaF_getlocalname (const Proto *func, int local_number,
+                                         int pc);
+
+
+#endif