contrib/wix/defines.wxi

simplemerge: enable option to resolve conflicts one way

With this change, the simplemerge algorithm grows an option to only
return the local or the other hunk in a conflicting region.

<Include>

  <?if $(var.Platform) = "x64" ?>
    <?define IsX64 = yes ?>
  <?else?>
    <?define IsX64 = no ?>
  <?endif?>

</Include>