Windows: Add lua52compat option to msvcbuild.bat.

Thanks to Gil Reis. #1366

src/msvcbuild.bat

@@ -7,6 +7,7 @@
 @rem
 @rem   nogc64        disable LJ_GC64 mode for x64
 @rem   debug         emit debug symbols
+@rem   lua52compat   enable extra Lua 5.2 extensions
 @rem   amalg         amalgamated build
 @rem   static        create static lib to statically link into your project
 @rem   mixed         create static lib to build a DLL in your project
@@ -101,6 +102,10 @@ buildvm -m folddef -o lj_folddef.h lj_opt_fold.c
 @set LJDYNBUILD=%LJDYNBUILD_DEBUG%
 @set LJLINKTYPE=%LJLINKTYPE_DEBUG%
 :NODEBUG
+@if "%1" neq "lua52compat" goto :NOLUA52COMPAT
+@shift
+@set LJCOMPILE=%LJCOMPILE% /DLUAJIT_ENABLE_LUA52COMPAT
+:NOLUA52COMPAT
 @set LJCOMPILE=%LJCOMPILE% %LJCOMPILETARGET%
 @set LJLINK=%LJLINK% %LJLINKTYPE% %LJLINKTARGET%
 @if "%1"=="amalg" goto :AMALGDLL