ENH: Hide windows for processes run by RunSingleCommand.

Source/cmSystemTools.cxx

@@ -386,6 +386,7 @@ bool cmSystemTools::RunSingleCommand(
   cmsysProcess* cp = cmsysProcess_New();
   cmsysProcess_SetCommand(cp, &*argv.begin());
   cmsysProcess_SetWorkingDirectory(cp, dir);
+  cmsysProcess_SetOption(cp, cmsysProcess_Option_HideWindow, 1);
   cmsysProcess_SetTimeout(cp, timeout);
   cmsysProcess_Execute(cp);