how to detect OS architecture?

[Hugo Florentino]

GetNativeSystemInfo worked. Thanks!

The code ended being like this (it seems to be working both in x86 and 
x86_64):

import std.file: exists, getcwd;
import std.path: buildPath, dirName;
import std.string: format, toStringz;
import core.sys.windows.windows;

enum X86   = 0;
enum AMD64 = 9;
immutable static auto appname = "myapp";

extern(Windows) HANDLE ShellExecuteA(HWND, LPCSTR, LPCSTR, LPCSTR, 
LPCSTR, int);
extern(Windows) int MessageBoxA(HWND, LPCSTR, LPCSTR, UINT);

int main() {
   auto appath = getcwd();
   string appexe;
   SYSTEM_INFO env;
   GetNativeSystemInfo(&env);
   switch(env.wProcessorArchitecture) {
     case X86: appexe = buildPath(appath, appname ~ "32.exe"); break;
     case AMD64: appexe = buildPath(appath, appname ~ "64.exe"); break;
     default:
       MessageBoxA(null, "System architecture is not supported.", 
"Error", MB_ICONHAND + MB_OK);
       return -1;
   }
   if (exists(appexe)) {
     auto param = format(`/ini="%s"`, buildPath(appath, appname ~ 
".ini"));
     ShellExecuteA(null, "", toStringz(appexe), toStringz(param), "", 
SW_SHOWMAXIMIZED);
     scope(failure) return -2;
   }
   return 0;
}