Fix runtime library flags for MSVC debug builds.

Reported by igor725. #1127

src/msvcbuild.bat

@@ -15,6 +15,7 @@
 @rem Add more debug flags here, e.g. DEBUGCFLAGS=/DLUA_USE_APICHECK
 @set DEBUGCFLAGS=
 @set LJCOMPILE=cl /nologo /c /O2 /W3 /D_CRT_SECURE_NO_DEPRECATE /D_CRT_STDIO_INLINE=__declspec(dllexport)__inline
+@set LJDYNBUILD=/MD /DLUA_BUILD_AS_DLL
 @set LJLINK=link /nologo
 @set LJMT=mt /nologo
 @set LJLIB=lib /nologo /nodefaultlib
@@ -71,11 +72,12 @@ buildvm -m folddef -o lj_folddef.h lj_opt_fold.c
 @shift
 @set BUILDTYPE=debug
 @set LJCOMPILE=%LJCOMPILE% /Zi %DEBUGCFLAGS%
+@set LJDYNBUILD=/MDd /DLUA_BUILD_AS_DLL
 :NODEBUG
 @set LJLINK=%LJLINK% /%BUILDTYPE%
 @if "%1"=="amalg" goto :AMALGDLL
 @if "%1"=="static" goto :STATIC
-%LJCOMPILE% /MD /DLUA_BUILD_AS_DLL lj_*.c lib_*.c
+%LJCOMPILE% %LJDYNBUILD% lj_*.c lib_*.c
 @if errorlevel 1 goto :BAD
 %LJLINK% /DLL /out:%LJDLLNAME% lj_*.obj lib_*.obj
 @if errorlevel 1 goto :BAD
@@ -87,7 +89,7 @@ buildvm -m folddef -o lj_folddef.h lj_opt_fold.c
 @if errorlevel 1 goto :BAD
 @goto :MTDLL
 :AMALGDLL
-%LJCOMPILE% /MD /DLUA_BUILD_AS_DLL ljamalg.c
+%LJCOMPILE% %LJDYNBUILD% ljamalg.c
 @if errorlevel 1 goto :BAD
 %LJLINK% /DLL /out:%LJDLLNAME% ljamalg.obj lj_vm.obj
 @if errorlevel 1 goto :BAD