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/etc/lua.pc

@@ -0,0 +1,31 @@
+# lua.pc -- pkg-config data for Lua
+
+# vars from install Makefile
+
+# grep '^V=' ../Makefile
+V= 5.1
+# grep '^R=' ../Makefile
+R= 5.1.5
+
+# grep '^INSTALL_.*=' ../Makefile | sed 's/INSTALL_TOP/prefix/'
+prefix= /usr/local
+INSTALL_BIN= ${prefix}/bin
+INSTALL_INC= ${prefix}/include
+INSTALL_LIB= ${prefix}/lib
+INSTALL_MAN= ${prefix}/man/man1
+INSTALL_LMOD= ${prefix}/share/lua/${V}
+INSTALL_CMOD= ${prefix}/lib/lua/${V}
+
+# canonical vars
+exec_prefix=${prefix}
+libdir=${exec_prefix}/lib
+includedir=${prefix}/include
+
+Name: Lua
+Description: An Extensible Extension Language
+Version: ${R}
+Requires: 
+Libs: -L${libdir} -llua -lm
+Cflags: -I${includedir}
+
+# (end of lua.pc)