partially fix amd64 build on windows

Quality of this commit is questionable. I have no idea whether what I did
here makes sense or not.
But it compiles and runs (provided with hlsdk-xash3d amd64 build, which
is another story).

This omits necessary waf changes that are necessary to make a valid
64-bit build. Apparently it's not enough to just pass `-8` on Windows,
you also need to hack wscript to add `x64` target to MSVC. I'll do that
later when I figure out how.

This change is a precursor for RTX Vulkan effort --
VK_KHR_ray_tracing_pipeline and friends are only available on 64-bit
nvidia drivers (no idea about AMD, pls send GPUs onegai).

engine/client/cl_game.c

@@ -3945,7 +3945,7 @@ qboolean CL_LoadProgs( const char *name )
 
 	// a1ba: we need to check if client.dll has direct dependency on SDL2
 	// and if so, disable relative mouse mode
-#if XASH_WIN32
+#if XASH_WIN32 && !defined(XASH_64BIT)
 	if( ( clgame.client_dll_uses_sdl = COM_CheckLibraryDirectDependency( name, OS_LIB_PREFIX "SDL2." OS_LIB_EXT, false ) ) )
 	{
 		Con_Printf( S_NOTE "%s uses SDL2 for mouse input\n", name );